On Mon, 30 Jan 2017, Ondrej Zary wrote:
>
> Also tested two CD-ROM drives and they didn't work (machine hangs). They
> didn't work before and looks like they never worked with PDMA.
>
> The fundamental problem of PDMA is that it either transfers all required
> data or it breaks horribly. Even when I added timeouts to the possibly
> infinite loops (to avoid hangs), the chip remained in some bad state.
> Sometimes the 53C80 gated IRQ check triggers. This can be hopefully
> fixed but there is a HW limitation: if less than 128 bytes were read
> from SCSI device, they get lost in chip buffer (the 128B buffers don't
> swap until full).
>
The core driver allows wrapper drivers to inhibit PDMA e.g. for inbound
transfers smaller than 128 bytes. This is used by atari_scsi and sun3_scsi
(as you probably realize). For example, atari_scsi now uses
cmd->sc_data_direction to find out the direction of the DMA transfer.
> Fortunately, most of the requests for too much data are bogus. The
> problem is that generic_NCR5380_dma_xfer_len() uses cmd->transfersize
> which seems to be wrong. All other NCR5380 drivers use
> cmd->SCp.this_residual.
>
Yes, this always looked wrong to me. Much of the logic in
generic_NCR5380_dma_xfer_len() looks suspicious but I see you've addressed
this below. Thanks for looking into this.
> This is also why rescan-scsi-bus hangs (cmd->transfersize is 8192 but
> cmd->SCp.this_residual is only 96).
>
Great, that resolves that mystery.
> This quick fix allows CD-ROM and also rescan-scsi-bus to work.
> But it's not complete. Seems that we need:
> - fix pread and pwrite to terminate gracefully
mac_scsi.c probably offers a reasonable example of a PDMA transfer
algorithm that is resiliant to timeouts.
See also the cmd->device->borken logic in the core driver. This way,
generic_NCR5380_pread() or generic_NCR5380_pwrite() can inhibit further
PDMA to that device if need be.
> - something like atari_scsi_dma_xfer_len to allow DMA only for block
> commands
>
Are you trying to figure out which commands are going to disconnect during
a transfer? This is really a function of the firmware in the target; there
are no good heuristics AFAICT, so the PDMA algorithm has to be robust.
mac_scsi has to cope with this too.
Does the problem go away when you assign no IRQ? When instance->irq ==
NO_IRQ, the core driver will inhibit disconnect privileges.
> @@ -588,22 +619,19 @@ static inline int generic_NCR5380_pwrite(struct NCR5380_hostdata *hostdata,
> static int generic_NCR5380_dma_xfer_len(struct NCR5380_hostdata *hostdata,
> struct scsi_cmnd *cmd)
> {
> - int transfersize = cmd->transfersize;
> + int transfersize = cmd->SCp.this_residual;
>
> if (hostdata->flags & FLAG_NO_PSEUDO_DMA)
> return 0;
>
> - /* Limit transfers to 32K, for xx400 & xx406
> - * pseudoDMA that transfers in 128 bytes blocks.
> - */
> - if (transfersize > 32 * 1024 && cmd->SCp.this_residual &&
> - !(cmd->SCp.this_residual % transfersize))
> - transfersize = 32 * 1024;
> -
> /* 53C400 datasheet: non-modulo-128-byte transfers should use PIO */
> if (transfersize % 128)
> transfersize = 0;
>
> + /* Limit transfers to 32K */
> + if (transfersize > 32 * 1024)
> + transfersize = 32 * 1024;
> +
> return transfersize;
I would prefer to see,
#define G_NCR5380_DMA_MAX_SIZE 32768
...
return min(transfersize, G_NCR5380_DMA_MAX_SIZE);
Thanks.
--
> }
>
>
>
>
>
--
To unsubscribe from this list: send the line "unsubscribe linux-scsi" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
