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... -- -- 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
