linux-kernel - Re: Regarding AHCI_MAX_SG and (ATA_HORKAGE_MAX_SEC

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Date:	Wed, 10 Aug 2016 10:41:55 -0500
From:	David Milburn <dmilburn@...hat.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Tom Yan <tom.ty89@...il.com>, linux-ide@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Regarding AHCI_MAX_SG and (ATA_HORKAGE_MAX_SEC_1024)

Hi,

On 08/10/2016 10:14 AM, Tejun Heo wrote:
> Hello, Tom.
>
> On Wed, Aug 10, 2016 at 06:04:10PM +0800, Tom Yan wrote:
>> On 10 August 2016 at 11:26, Tejun Heo <tj@...nel.org> wrote:
>>> Hmmm.. why not?  The hardware limit is 64k and the driver is using a
>>
>> Is that referring to the maximum number of entries allowed in the
>> PRDT, Physical Region Descriptor Table (which is, more precisely,
>> 65535)?
>
> Yeap.
>
>>> Not necessarily.  A single sg entry can point to an area larger than
>>> PAGE_SIZE.
>>
>> You mean the 4MB limit of "Data Byte Count" in "DW3: Description
>> Information" of the PRDT? Is that what max_segment_size (which is set
>> to a general fallback of 65536:
>> http://lxr.free-electrons.com/ident?i=dma_get_max_seg_size) is about
>> in this case?
>
> Ah, ahci isn't setting the hardware limit properly but yeah that's the
> maximum segment size.
>
>> And my point was, it will be a multiple of 168 anyway, if 1344 is just
>> an example.
>>
>>> As written above, that probably makes the ahci command table size
>>> nicely aligned.
>>
>> I think that's what bothers me ultimately, cause I don't see how 168
>> makes it (more) nicely aligned (or even, aligned to what?).
>
> Hmmm... Looked at the sizes and they don't seem to align to anything
> meaningful.  No idea.

The 168 makes AHCI_CMD_TBL_SZ equal to 2816

AHCI_CMD_TBL_SZ = AHCI_CMD_TBL_HDR_SZ + (AHCI_MAX_SG * 16)
AHCI_CMD_TBL_SZ = 128 + (168 * 16)

I think if you add in AHCI_CMD_SLOT_SZ (1024) and AHCI_RX_FIS_SZ (256)
the DMA is 4K aligned, I think that is where the 168 came from.

Thanks,
David