linux-kernel - Re: [PATCH 02/15] ARM: pxa: add dma slave map

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Date:   Wed, 4 Apr 2018 12:18:58 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Robert Jarzmik <robert.jarzmik@...e.fr>
Cc:     Daniel Mack <daniel@...que.org>,
        Haojian Zhuang <haojian.zhuang@...il.com>,
        Vinod Koul <vinod.koul@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dmaengine@...r.kernel.org
Subject: Re: [PATCH 02/15] ARM: pxa: add dma slave map

On Tue, Apr 3, 2018 at 10:19 PM, Robert Jarzmik <robert.jarzmik@...e.fr> wrote:
> ... chop chop removing unneeded recipients ....
>
> Arnd Bergmann <arnd@...db.de> writes:
>
>> Ok, I looked at the driver in more detail now and found the scary parts.
>> So it's using the async DMA interface to do synchronous DMA in
>> interrupt context in order to transfer the rx data faster than an readsl()
>> would, correct?
> That's correct, at least for the smc91x.
>
>> It still feels odd to me that there is an entry in the slave map for
>> a device that does not have a request line. However, it also seems
>> that the entire code in those two drivers that deals with DMA is specific
>> to PXA anyway, so maybe it can be done differently: instead of
>> calling dma_request_slave_channel_compat() or dma_request_chan()
>> with a fake request line, how about calling dma_request_channel()
>> with an NULL filter function and data, and have the driver handle
>> the empty data case the same way as the rq=-1 case today?
> Okay, in this case :
>  - the channel priority cannot be passed anymore

Right, but it could just always use a static priority, right?

>  - and I don't see how this can work :
>    dma_request_channel()
>      __dma_request_channel()
>        find_candidate()
>          private_candidate(mask, device, fn, fn_param);
>            /* Here, fn == NULL and fn_param == NULL as per your proposal */
>
>    This function will find the first available dma channel, all right, but
>    no function will be called in pxa_dma driver, and therefore the last
>    requestor of the channel will be used, which is bad.

Can't you just reset those in pxad_free_chan_resources()?

       Arnd