[PATCH] ata: pata_macio: Fix max_segment_size with PAGE_SIZE == 64K

[Michael Ellerman]

The pata_macio driver advertises a max_segment_size of 0xff00, because
the hardware doesn't cope with requests >= 64K.

However the SCSI core requires max_segment_size to be at least
PAGE_SIZE, which is a problem for pata_macio when the kernel is built
with 64K pages.

In older kernels the SCSI core would just increase the segment size to
be equal to PAGE_SIZE, however since the commit tagged below it causes a
warning and the device fails to probe:

  WARNING: CPU: 0 PID: 26 at block/blk-settings.c:202 .blk_validate_limits+0x2f8/0x35c
  CPU: 0 PID: 26 Comm: kworker/u4:1 Not tainted 6.10.0-rc1 #1
  Hardware name: PowerMac7,2 PPC970 0x390202 PowerMac
  ...
  NIP .blk_validate_limits+0x2f8/0x35c
  LR  .blk_alloc_queue+0xc0/0x2f8
  Call Trace:
    .blk_alloc_queue+0xc0/0x2f8
    .blk_mq_alloc_queue+0x60/0xf8
    .scsi_alloc_sdev+0x208/0x3c0
    .scsi_probe_and_add_lun+0x314/0x52c
    .__scsi_add_device+0x170/0x1a4
    .ata_scsi_scan_host+0x2bc/0x3e4
    .async_port_probe+0x6c/0xa0
    .async_run_entry_fn+0x60/0x1bc
    .process_one_work+0x228/0x510
    .worker_thread+0x360/0x530
    .kthread+0x134/0x13c
    .start_kernel_thread+0x10/0x14
  ...
  scsi_alloc_sdev: Allocation failure during SCSI scanning, some SCSI devices might not be configured

Although the hardware can't cope with a 64K segment, the driver
already deals with that internally by splitting large requests in
pata_macio_qc_prep(). That is how the driver has managed to function
until now on 64K kernels.

So fix the driver to advertise a max_segment_size of 64K, which avoids
the warning and keeps the SCSI core happy.

Fixes: afd53a3d8528 ("scsi: core: Initialize scsi midlayer limits before allocating the queue")
Reported-by: Guenter Roeck <linux at roeck-us.net>
Closes: https://lore.kernel.org/all/ce2bf6af-4382-4fe1-b392-cc6829f5ceb2@roeck-us.net/
Reported-by: Doru Iorgulescu <doru.iorgulescu1 at gmail.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218858
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
Reviewed-by: Christoph Hellwig <hch at lst.de>
---
 drivers/ata/pata_macio.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/ata/pata_macio.c b/drivers/ata/pata_macio.c
index 817838e2f70e..3cb455a32d92 100644
--- a/drivers/ata/pata_macio.c
+++ b/drivers/ata/pata_macio.c
@@ -915,10 +915,13 @@ static const struct scsi_host_template pata_macio_sht = {
 	.sg_tablesize		= MAX_DCMDS,
 	/* We may not need that strict one */
 	.dma_boundary		= ATA_DMA_BOUNDARY,
-	/* Not sure what the real max is but we know it's less than 64K, let's
-	 * use 64K minus 256
+	/*
+	 * The SCSI core requires the segment size to cover at least a page, so
+	 * for 64K page size kernels this must be at least 64K. However the
+	 * hardware can't handle 64K, so pata_macio_qc_prep() will split large
+	 * requests.
 	 */
-	.max_segment_size	= MAX_DBDMA_SEG,
+	.max_segment_size	= SZ_64K,
 	.device_configure	= pata_macio_device_configure,
 	.sdev_groups		= ata_common_sdev_groups,
 	.can_queue		= ATA_DEF_QUEUE,
-- 
2.45.1