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?). > > When you said, the "PDMA operation hangs waiting forever", I figured that > you had hit an infinite loop in NCR5380_pread()... but now I'm lost. > > My main concern here is to confirm that I didn't break anything e.g. with > patch 24 or 41. It would be nice to know that this hang is not the result > of a new bug. > > > 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: --- 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 */ + 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 -- Ondrej Zary -- 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
