Re: [PATCH v3 2/5] tty: Introduce SER_RS485_SOFTWARE read-only flag for struct serial_rs485

[Peter Hurley]

On 11/12/2015 08:26 PM, Andy Shevchenko wrote:
> On Fri, Nov 13, 2015 at 3:11 AM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
>> On 11/12/2015 07:41 PM, Andy Shevchenko wrote:
>>> On Thu, Nov 12, 2015 at 10:22 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
>>>> On 11/12/2015 02:57 PM, One Thousand Gnomes wrote:
>>>
>>>> An illustrative (kernel-space) example is the mess that is dmaengine_pause().
>>>> Some DMA implementations provide the means to stop and restart DMA without
>>>> losing data and some DMA implementations do not. Unfortunately, some
>>>> advertise they support dmaengine_pause() but only for lossy uses like audio.
>>>> Because the api hides this, the query interface for pause support is
>>>> useless.
>>>
>>> The DMA pause() call means only pause with possibility to resume.
>>> There is a resume() call as well. Any driver which treats pause() as a
>>> complete stop is buggy driver and should be fixed.
>>
>> How about pause _without_ the possibility to resume?
>>
>> https://groups.google.com/d/msg/linux.kernel/Abe0hfGcgsw/H0se55wC558J
> 
> Briefly what I got from the thread that Russell shows similar view on
> the API, so that's why he was objecting to add pause/resume calls for
> a specific hardware.

Not quite.

That dmaengine driver (omap-dma) advertises that it supports pause() via
dma_get_slave_caps(). And if you call it with a cyclic channel it will pause.
However, if you call dmaengine_pause() with a slave channel it returns an error
*because the hardware can't actually meet the criteria for dmaengine_pause()*
which is pause()/resume() without data loss.

IOW, there is no method of determining a priori if dmaengine_pause() will
categorically fail for a given transfer type.

Regards,
Peter Hurley
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