linux-kernel - Re: [PATCHv2 1/7] block: check for valid bio while splitting

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Message-ID: <d9116c88-4098-46a7-8cbc-c900576a5da3@acm.org>
Date: Wed, 13 Aug 2025 13:41:49 -0700
From: Bart Van Assche <bvanassche@....org>
To: Keith Busch <kbusch@...nel.org>, Keith Busch <kbusch@...a.com>
Cc: linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, snitzer@...nel.org, axboe@...nel.dk,
 dw@...idwei.uk, brauner@...nel.org, Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCHv2 1/7] block: check for valid bio while splitting

On 8/13/25 1:06 PM, Keith Busch wrote:
> But I can't make that change because many scsi devices don't set the dma
> alignment and get the default 511 value. This is fine for the memory
> address offset, but the lengths sent for various inquriy commands are
> much smaller, like 4 and 32 byte lengths. That length wouldn't pass the
> dma alignment granularity, so I think the default value is far too
> conservative. Does the address start size need to be a different limit
> than minimum length? I feel like they should be the same, but maybe
> that's just an nvme thing.

Hi Keith,

Maybe I misunderstood your question. It seems to me that the SCSI core
sets the DMA alignment by default to four bytes. From
drivers/scsi/hosts.c:

	/* 32-byte (dword) is a common minimum for HBAs. */
	if (sht->dma_alignment)
		shost->dma_alignment = sht->dma_alignment;
	else
		shost->dma_alignment = 3;

Bart.