On Tue, 24 Nov 2015, Ondrej Zary wrote:
On Tuesday 24 November 2015 10:13:17 Finn Thain wrote:
On Tue, 24 Nov 2015, Ondrej Zary wrote:
On Tuesday 24 November 2015, Finn Thain wrote:
On Mon, 23 Nov 2015, Ondrej Zary wrote:
PDMA seems to be broken in multiple ways. NCR5380_pread cannot
process less than 128 bytes. In fact, 53C400 datasheet says that
it's HW limitation: non-modulo-128-byte transfers should use
PIO.
Adding
transfersize = round_down(transfersize, 128);
to generic_NCR5380_dma_xfer_len() improves the situation a bit.
After modprobe, some small reads (8, 4, 24 and 64 bytes) are
done using PIO, then eight 512-byte reads using PDMA and then it
fails on a 254-byte read. First 128 bytes are read using PDMA
and the next PDMA operation hangs waiting forever for the host
buffer to be ready.
A 128-byte PDMA receive followed by 126-byte PDMA receive? I don't
see how that is possible given round_down(126, 128) == 0. Was this
the actual 'len' argument to NCR5380_pread() in g_NCR5380.c?
No 126-byte PDMA. The 126 bytes were probably lost (or mixed with
the next read?).
[...]
The next read was also 254 bytes so another 128-byte PDMA transfer.
Then modified NCR5380_information_transfer() to transfer the
remaining data (126 bytes in this case) using PIO. It did not help,
the next PDMA transfer failed too.
AFAICT, no change to NCR5380_information_transfer() should be needed.
It was always meant to cope with the need to split a transfer between
(P)DMA and PIO.
Instead of fixing split transfers, simply forced everything
non-modulo-128 to PIO:
The need to split a transfer arises from early chip errata relating to DMA
and the workarounds for them (see the comments in the source). That's why
I believe that the driver was meant to be cope with this. But I don't have
any experimental evidence for it.
I'm almost certain that these errata aren't applicable to your hardware.
So I don't have any reason to think that your card will allow part of a
transfer to be performed with PDMA and the rest with PIO. So I don't
really object to the patch.
But I don't understand the need for it either: I have no idea what state
the driver, chip and scsi bus were in when the 126-byte PIO transfer
failed. If the PIO transfer didn't succeed then the entire command should
have failed.
--- a/drivers/scsi/g_NCR5380.c
+++ b/drivers/scsi/g_NCR5380.c
@@ -703,6 +703,10 @@ static int generic_NCR5380_dma_xfer_len(struct scsi_cmnd *cmd)
!(cmd->SCp.this_residual % transfersize))
transfersize = 32 * 1024;
+ /* 53C400 datasheet: non-modulo-128-byte transfers should use PIO */
Do you have a download link for this datasheet?
+ if (transfersize % 128)
+ transfersize = 0;
+
return transfersize;
}
It seems to work and greatly improves performance:
# hdparm -t --direct /dev/sdb
/dev/sdb:
Timing O_DIRECT disk reads: 4 MB in 4.84 seconds = 846.15 kB/sec
Sounds about right...
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