linux-kernel - Re: [PATCH 2/2] DMAENGINE: generic channel status

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Date:	Thu, 25 Mar 2010 15:00:04 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Guennadi Liakhovetski <g.liakhovetski@....de>
Cc:	Linus Walleij <linus.walleij@...ricsson.com>,
	linux-kernel@...r.kernel.org,
	Maciej Sosnowski <maciej.sosnowski@...el.com>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Pavel Machek <pavel@....cz>, Li Yang <leoli@...escale.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Haavard Skinnemoen <haavard.skinnemoen@...el.com>,
	Magnus Damm <damm@...nsource.se>,
	Liam Girdwood <lrg@...mlogic.co.uk>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Joe Perches <joe@...ches.com>,
	Roland Dreier <rdreier@...co.com>
Subject: Re: [PATCH 2/2] DMAENGINE: generic channel status

On Thu, Mar 25, 2010 at 1:58 PM, Guennadi Liakhovetski
<g.liakhovetski@....de> wrote:
> On Thu, 25 Mar 2010, Linus Walleij wrote:
>
>> Convert the device_is_tx_complete() operation on the
>> DMA engine to a generic device_tx_status()operation which
>> can return three states, DMA_TX_RUNNING, DMA_TX_COMPLETE,
>> DMA_TX_PAUSED.
>>
[..]
> General: you converted all drivers to the new .device_tx_status() API, but
> since they don't implement "residue," you left it uninitialised
> everywhere. Wouldn't it be better to set it to 0 or total length,
> depending on the complete / not complete status?

Agree that it should not be uninitialized.  At the same time I do not
want to require drivers that don't need it to go through the hassle of
looking up a byte count, so perhaps all but the drivers that want this
support can return a 'max byte count'??

>
>> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
>> index 0731802..c9f2c67 100644
>> --- a/include/linux/dmaengine.h
>> +++ b/include/linux/dmaengine.h
>> @@ -40,11 +40,13 @@ typedef s32 dma_cookie_t;
>>   * enum dma_status - DMA transaction status
>>   * @DMA_SUCCESS: transaction completed successfully
>>   * @DMA_IN_PROGRESS: transaction not yet processed
>> + * @DMA_PAUSED: transaction is paused
>>   * @DMA_ERROR: transaction failed
>>   */
>>  enum dma_status {
>>       DMA_SUCCESS,
>>       DMA_IN_PROGRESS,
>> +     DMA_PAUSED,
>>       DMA_ERROR,
>>  };
>>
>> @@ -249,6 +251,21 @@ struct dma_async_tx_descriptor {
>>  };
>>
>>  /**
>> + * struct dma_tx_state - filled in to report the status of
>> + * a transfer.
>> + * @last: last completed DMA cookie
>> + * @used: last issued DMA cookie (i.e. the one in progress)
>> + * @residue: the remaining number of bytes left to transmit
>> + *   on the selected transfer for states DMA_IN_PROGRESS and
>> + *   DMA_PAUSED if this is implemented in the driver, else 0
>> + */
>> +struct dma_tx_state {
>> +     dma_cookie_t last;
>> +     dma_cookie_t used;
>> +     u32 residue;
>
> In the original proposal by Dan Williams the last member was "unsigned
> long pos." I don't think, even on 64-bit systems anyone would kick off a >
> 4GB transfer, but who knows... And - I don't particularly like the name
> "pos," but I do like the idea of returning bytes transfered better, than
> bytes left. Can we change this?

Things like copy_from_user() and the scsi subsystem return residue, so
only for consistency with other areas would I side with the "bytes
left" camp.

Ack to all your other comments.

--
Dan
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