23.8 xxx_isa_probe

This function probes if a device is present. If the driver supports auto-detection of some part of device configuration (such as interrupt vector or memory address) this auto-detection must be done in this routine.

As for any other bus, if the device cannot be detected or is detected but failed the self-test or some other problem happened then it returns a positive value of error. The value ENXIO must be returned if the device is not present. Other error values may mean other conditions. Zero or negative values mean success. Most of the drivers return zero as success.

The negative return values are used when a PnP device supports multiple interfaces. For example, an older compatibility interface and a newer advanced interface which are supported by different drivers. Then both drivers would detect the device. The driver which returns a higher value in the probe routine takes precedence (in other words, the driver returning 0 has highest precedence, one returning -1 is next, one returning -2 is after it and so on). In result the devices which support only the old interface will be handled by the old driver (which should return -1 from the probe routine) while the devices supporting the new interface as well will be handled by the new driver (which should return 0 from the probe routine).

The device descriptor struct xxx_softc is allocated by the system before calling the probe routine. If the probe routine returns an error the descriptor will be automatically deallocated by the system. So if a probing error occurs the driver must make sure that all the resources it used during probe are deallocated and that nothing keeps the descriptor from being safely deallocated. If the probe completes successfully the descriptor will be preserved by the system and later passed to the routine xxx_isa_attach(). If a driver returns a negative value it can not be sure that it will have the highest priority and its attach routine will be called. So in this case it also must release all the resources before returning and if necessary allocate them again in the attach routine. When xxx_isa_probe() returns 0 releasing the resources before returning is also a good idea and a well-behaved driver should do so. But in cases where there is some problem with releasing the resources the driver is allowed to keep resources between returning 0 from the probe routine and execution of the attach routine.

A typical probe routine starts with getting the device descriptor and unit:

             struct xxx_softc *sc = device_get_softc(dev);
              int unit = device_get_unit(dev);
              int pnperror;
              int error = 0;
    
              sc->dev = dev; /* link it back */
              sc->unit = unit;       

Then check for the PnP devices. The check is carried out by a table containing the list of PnP IDs supported by this driver and human-readable descriptions of the device models corresponding to these IDs.

            pnperror=ISA_PNP_PROBE(device_get_parent(dev), dev,
            xxx_pnp_ids); if(pnperror == ENXIO) return ENXIO;
           

The logic of ISA_PNP_PROBE is the following: If this card (device unit) was not detected as PnP then ENOENT will be returned. If it was detected as PnP but its detected ID does not match any of the IDs in the table then ENXIO is returned. Finally, if it has PnP support and it matches on of the IDs in the table, 0 is returned and the appropriate description from the table is set by device_set_desc().

If a driver supports only PnP devices then the condition would look like:

              if(pnperror != 0)
                  return pnperror;       

No special treatment is required for the drivers which do not support PnP because they pass an empty PnP ID table and will always get ENXIO if called on a PnP card.

The probe routine normally needs at least some minimal set of resources, such as I/O port number to find the card and probe it. Depending on the hardware the driver may be able to discover the other necessary resources automatically. The PnP devices have all the resources pre-set by the PnP subsystem, so the driver does not need to discover them by itself.

Typically the minimal information required to get access to the device is the I/O port number. Then some devices allow to get the rest of information from the device configuration registers (though not all devices do that). So first we try to get the port start value:

     sc->port0 = bus_get_resource_start(dev,
            SYS_RES_IOPORT, 0 /*rid*/); if(sc->port0 == 0) return ENXIO;
           

The base port address is saved in the structure softc for future use. If it will be used very often then calling the resource function each time would be prohibitively slow. If we do not get a port we just return an error. Some device drivers can instead be clever and try to probe all the possible ports, like this:

              
              /* table of all possible base I/O port addresses for this device */
              static struct xxx_allports {
                  u_short port; /* port address */
                  short used; /* flag: if this port is already used by some unit */
              } xxx_allports = {
                  { 0x300, 0 },
                  { 0x320, 0 },
                  { 0x340, 0 },
                  { 0, 0 } /* end of table */
              };
    
              ...
              int port, i;
              ...
    
              port =  bus_get_resource_start(dev, SYS_RES_IOPORT, 0 /*rid*/);
              if(port !=0 ) {
                  for(i=0; xxx_allports[i].port!=0; i++) {
                      if(xxx_allports[i].used || xxx_allports[i].port != port)
                          continue;
    
                      /* found it */
                      xxx_allports[i].used = 1;
                      /* do probe on a known port */
                      return xxx_really_probe(dev, port);
                  }
                  return ENXIO; /* port is unknown or already used */
              }
    
              /* we get here only if we need to guess the port */
              for(i=0; xxx_allports[i].port!=0; i++) {
                  if(xxx_allports[i].used)
                      continue;
    
                  /* mark as used - even if we find nothing at this port
                   * at least we won't probe it in future
                   */
                   xxx_allports[i].used = 1;
    
                  error = xxx_really_probe(dev, xxx_allports[i].port);
                  if(error == 0) /* found a device at that port */
                      return 0;
              }
              /* probed all possible addresses, none worked */
              return ENXIO;

Of course, normally the driver's identify() routine should be used for such things. But there may be one valid reason why it may be better to be done in probe(): if this probe would drive some other sensitive device crazy. The probe routines are ordered with consideration of the sensitive flag: the sensitive devices get probed first and the rest of the devices later. But the identify() routines are called before any probes, so they show no respect to the sensitive devices and may upset them.

Now, after we got the starting port we need to set the port count (except for PnP devices) because the kernel does not have this information in the configuration file.

              
             if(pnperror /* only for non-PnP devices */
             && bus_set_resource(dev, SYS_RES_IOPORT, 0, sc->port0, 
             XXX_PORT_COUNT)<0)
                 return ENXIO;

Finally allocate and activate a piece of port address space (special values of start and end mean ``use those we set by bus_set_resource()''):

              sc->port0_rid = 0;
              sc->port0_r = bus_alloc_resource(dev, SYS_RES_IOPORT,  
              &sc->port0_rid,
                  /*start*/ 0, /*end*/ ~0, /*count*/ 0, RF_ACTIVE);
    
              if(sc->port0_r == NULL)
                  return ENXIO;

Now having access to the port-mapped registers we can poke the device in some way and check if it reacts like it is expected to. If it does not then there is probably some other device or no device at all at this address.

Normally drivers do not set up the interrupt handlers until the attach routine. Instead they do probes in the polling mode using the DELAY() function for timeout. The probe routine must never hang forever, all the waits for the device must be done with timeouts. If the device does not respond within the time it is probably broken or misconfigured and the driver must return error. When determining the timeout interval give the device some extra time to be on the safe side: although DELAY() is supposed to delay for the same amount of time on any machine it has some margin of error, depending on the exact CPU.

If the probe routine really wants to check that the interrupts really work it may configure and probe the interrupts too. But that is not recommended.

              
              /* implemented in some very device-specific way */
              if(error = xxx_probe_ports(sc))
                  goto bad; /* will deallocate the resources before returning */
           

The function xxx_probe_ports() may also set the device description depending on the exact model of device it discovers. But if there is only one supported device model this can be as well done in a hardcoded way. Of course, for the PnP devices the PnP support sets the description from the table automatically.

              if(pnperror)
                  device_set_desc(dev, "Our device model 1234");
           

Then the probe routine should either discover the ranges of all the resources by reading the device configuration registers or make sure that they were set explicitly by the user. We will consider it with an example of on-board memory. The probe routine should be as non-intrusive as possible, so allocation and check of functionality of the rest of resources (besides the ports) would be better left to the attach routine.

The memory address may be specified in the kernel configuration file or on some devices it may be pre-configured in non-volatile configuration registers. If both sources are available and different, which one should be used? Probably if the user bothered to set the address explicitly in the kernel configuration file they know what they are doing and this one should take precedence. An example of implementation could be:

              
              /* try to find out the config address first */
              sc->mem0_p = bus_get_resource_start(dev, SYS_RES_MEMORY, 0 /*rid*/);
              if(sc->mem0_p == 0) { /* nope, not specified by user */
                  sc->mem0_p = xxx_read_mem0_from_device_config(sc);
    
    
              if(sc->mem0_p == 0)
                      /* can't get it from device config registers either */
                      goto bad;
              } else {
                  if(xxx_set_mem0_address_on_device(sc) < 0)
                      goto bad; /* device does not support that address */
              }
    
              /* just like the port, set the memory size,
               * for some devices the memory size would not be constant
               * but should be read from the device configuration registers instead
               * to accommodate different models of devices. Another option would
               * be to let the user set the memory size as "msize" configuration
               * resource which will be automatically handled by the ISA bus.
               */
               if(pnperror) { /* only for non-PnP devices */
                  sc->mem0_size = bus_get_resource_count(dev, SYS_RES_MEMORY, 0 /*rid*/);
                  if(sc->mem0_size == 0) /* not specified by user */
                      sc->mem0_size = xxx_read_mem0_size_from_device_config(sc);
    
                  if(sc->mem0_size == 0) {
                      /* suppose this is a very old model of device without
                       * auto-configuration features and the user gave no preference,
                       * so assume the minimalistic case
                       * (of course, the real value will vary with the driver)
                       */
                      sc->mem0_size = 8*1024;
                  }
    
                  if(xxx_set_mem0_size_on_device(sc) < 0)
                      goto bad; /* device does not support that size */
    
                  if(bus_set_resource(dev, SYS_RES_MEMORY, /*rid*/0,
                          sc->mem0_p, sc->mem0_size)<0)
                      goto bad;
              } else {
                  sc->mem0_size = bus_get_resource_count(dev, SYS_RES_MEMORY, 0 /*rid*/);
              }       

Resources for IRQ and DRQ are easy to check by analogy.

If all went well then release all the resources and return success.

              xxx_free_resources(sc);
              return 0;

Finally, handle the troublesome situations. All the resources should be deallocated before returning. We make use of the fact that before the structure softc is passed to us it gets zeroed out, so we can find out if some resource was allocated: then its descriptor is non-zero.

              bad:
    
              xxx_free_resources(sc);
              if(error)
                    return error;
              else /* exact error is unknown */
                  return ENXIO;

That would be all for the probe routine. Freeing of resources is done from multiple places, so it is moved to a function which may look like:

    static void
               xxx_free_resources(sc)
                  struct xxx_softc *sc;
              {
                  /* check every resource and free if not zero */
    
                  /* interrupt handler */
                  if(sc->intr_r) {
                      bus_teardown_intr(sc->dev, sc->intr_r, sc->intr_cookie);
                      bus_release_resource(sc->dev, SYS_RES_IRQ, sc->intr_rid,
                          sc->intr_r);
                      sc->intr_r = 0;
                  }
    
                  /* all kinds of memory maps we could have allocated */
                  if(sc->data_p) {
                      bus_dmamap_unload(sc->data_tag, sc->data_map);
                      sc->data_p = 0;
                  }
                   if(sc->data) { /* sc->data_map may be legitimately equal to 0 */
                      /* the map will also be freed */
                      bus_dmamem_free(sc->data_tag, sc->data, sc->data_map);
                      sc->data = 0;
                  }
                  if(sc->data_tag) {
                      bus_dma_tag_destroy(sc->data_tag);
                      sc->data_tag = 0;
                  }
    
                  ... free other maps and tags if we have them ...
    
                  if(sc->parent_tag) {
                      bus_dma_tag_destroy(sc->parent_tag);
                      sc->parent_tag = 0;
                  }
    
                  /* release all the bus resources */
                  if(sc->mem0_r) {
                      bus_release_resource(sc->dev, SYS_RES_MEMORY, sc->mem0_rid,
                          sc->mem0_r);
                      sc->mem0_r = 0;
                  }
                  ...
                  if(sc->port0_r) {
                      bus_release_resource(sc->dev, SYS_RES_IOPORT, sc->port0_rid,
                          sc->port0_r);
                      sc->port0_r = 0;
                  }
              }