Re: [dm-devel] [PATCH v8 1/9] block: Introduce queue limits for copy-offload support

From: Damien Le Moal <damien.lemoal@opensource.wdc.com>
To: Nitesh Shetty <nj.shetty@samsung.com>
Cc: linux-nvme@lists.infradead.org, dm-devel@redhat.com,
	Christoph Hellwig <hch@lst.de>, Alasdair Kergon <agk@redhat.com>,
	Sagi Grimberg <sagi@grimberg.me>,
	gost.dev@samsung.com, nitheshshetty@gmail.com,
	James Smart <james.smart@broadcom.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Anuj Gupta <anuj20.g@samsung.com>,
	Mike Snitzer <snitzer@kernel.org>,
	ming.lei@redhat.com, linux-block@vger.kernel.org,
	Keith Busch <kbusch@kernel.org>,
	bvanassche@acm.org, Jens Axboe <axboe@kernel.dk>,
	Christian Brauner <brauner@kernel.org>,
	joshi.k@samsung.com, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [dm-devel] [PATCH v8 1/9] block: Introduce queue limits for copy-offload support
Date: Wed, 29 Mar 2023 21:24:11 +0900	[thread overview]
Message-ID: <03c647ff-3c4f-a810-12c4-06a9dc62c90e@opensource.wdc.com> (raw)
In-Reply-To: <20230329104142.GA11932@green5>

On 3/29/23 19:41, Nitesh Shetty wrote:
>>> +What:		/sys/block/<disk>/queue/copy_max_bytes
>>> +Date:		November 2022
>>> +Contact:	linux-block@vger.kernel.org
>>> +Description:
>>> +		[RW] While 'copy_max_bytes_hw' is the hardware limit for the
>>> +		device, 'copy_max_bytes' setting is the software limit.
>>> +		Setting this value lower will make Linux issue smaller size
>>> +		copies from block layer.
>>
>> 		This is the maximum number of bytes that the block
>>                 layer will allow for a copy request. Must be smaller than
>>                 or equal to the maximum size allowed by the hardware indicated
> 
> Looks good.  Will update in next version. We took reference from discard. 
> 
>> 		by copy_max_bytes_hw. Write 0 to use the default kernel
>> 		settings.
>>
> 
> Nack, writing 0 will not set it to default value. (default value is
> copy_max_bytes = copy_max_bytes_hw)

It is trivial to make it work that way, which would match how max_sectors_kb
works. Write 0 to return copy_max_bytes being equal to the default
copy_max_bytes_hw.

The other possibility that is also interesting is "write 0 to disable copy
offload and use emulation". This one may actually be more useful.

> 
>>> +
>>> +
>>> +What:		/sys/block/<disk>/queue/copy_max_bytes_hw
>>> +Date:		November 2022
>>> +Contact:	linux-block@vger.kernel.org
>>> +Description:
>>> +		[RO] Devices that support offloading copy functionality may have
>>> +		internal limits on the number of bytes that can be offloaded
>>> +		in a single operation. The `copy_max_bytes_hw`
>>> +		parameter is set by the device driver to the maximum number of
>>> +		bytes that can be copied in a single operation. Copy
>>> +		requests issued to the device must not exceed this limit.
>>> +		A value of 0 means that the device does not
>>> +		support copy offload.
>>
>> 		[RO] This is the maximum number of kilobytes supported in a
>>                 single data copy offload operation. A value of 0 means that the
>> 		device does not support copy offload.
>>
> 
> Nack, value is in bytes. Same as discard.

Typo. I meant Bytes. Your text is too long an too convoluted, so unclear.

-- 
Damien Le Moal
Western Digital Research

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