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

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Date:   Tue, 03 Apr 2018 22:19:33 +0200
From:   Robert Jarzmik <robert.jarzmik@...e.fr>
To:     Arnd Bergmann <arnd@...db.de>
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

... 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
 - 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.

>> If you think it's worth it, what is the driving benefit behind ?
> It seems a bit cleaner to only register the tables for the dma lines that
> are actually present on a given chip.
Okay, let's do this.

Cheers.

-- 
Robert