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

[Nitesh Shetty]

On Wed, Mar 29, 2023 at 09:24:11PM +0900, Damien Le Moal wrote:
> On 3/29/23 19:41, Nitesh Shetty wrote:
> >>> +What:		/sys/block/<disk>/queue/copy_max_bytes
> >>> +Date:		November 2022
> >>> +Contact:	linux-block@xxxxxxxxxxxxxxx
> >>> +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.
>

We were following discard implementation.
I feel now both options are good. We can finalize based on community feedback.

> > 
> >>> +
> >>> +
> >>> +What:		/sys/block/<disk>/queue/copy_max_bytes_hw
> >>> +Date:		November 2022
> >>> +Contact:	linux-block@xxxxxxxxxxxxxxx
> >>> +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.
>
Acked, will update in next version

> -- 
> Damien Le Moal
> Western Digital Research
> 
>