On 08/20/2013 12:36 PM, Michael Schmitz wrote:
Tuomas,
I guess those two bytes have something to do with target
(re)selection... There are two different ways of attempting to read
these bytes in the code, one is reading them from the FIFO byte by
byte, and the other way is to setup a DMA read (write from device).
The two different schemes are used in two different functions, and I
do not understand why one method would be preferred over the other
when comparing these functions.
What the driver attempts to read on reselection are the tag bytes that
were used in the previous disconnect. These have to match otherwise we
are not reselecting the command we need.
No idea which functions you refer to regarding PIO (reading from
FIFO). Anyway, the DMA will read from the same FIFO that the CPU
would, if I understand DMA correctly. There really should be no
difference-
esp.scsi.c:1058 versus esp.scsi.c:1120
I think I managed to find these two bytes; They are not ready for
DMA, but are sitting in the FIFO waiting to be read.
So the loop before the "IRQ2 timeout" is waiting for an interrupt
that would tell that the DMA transfer has finished.
The loop checks whether the ESP status indicates the transfer is done
- if the bytes are still stuck in the FIFO I'd guess the DMA does not
bother to transfer for such small transfer counts.
In:
http://pdf.datasheetcatalog.com/datasheet/AdvancedMicroDevices/mXxxsy.pdf
page 42: Enable Selection/Reselection Command (Command Code 44H/C4H)
explains why the bytes might end up in FIFO; The chip DMA got disabled
at some point. Also, you need to explicitly issue a specific (don't know
which, yet) command to let the DMA access the FIFO.
The DMA transfer function can be changed to fetch less than seven
bytes from the FIFO by PIO instead of setting up DMA transfer (just
poll the FIFO after sending the command to the ESP, instead of first
setting up the DMA then sending the command),
I'd rather find out why and where the DMA is disabled before we resort
to PIO. It is also possible that the chip interrupt is cleared
prematurely by reading the interrupt register, in which case the
interrupt is not seen by The Loop.
-Tuomas
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