linux-kernel - Re: [PATCH v2] serial: imx: adapt rx buffer and dma periods

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Date:   Thu, 19 Sep 2019 14:00:30 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Philipp Puschmann <philipp.puschmann@...ix.com>
Cc:     festevam@...il.com, fugang.duan@....com,
        linux-serial@...r.kernel.org, gregkh@...uxfoundation.org,
        s.hauer@...gutronix.de, linux-kernel@...r.kernel.org,
        linux-imx@....com, kernel@...gutronix.de, jslaby@...e.com,
        yibin.gong@....com, shawnguo@...nel.org,
        linux-arm-kernel@...ts.infradead.org, l.stach@...gutronix.de
Subject: Re: [PATCH v2] serial: imx: adapt rx buffer and dma periods

On Thu, Sep 19, 2019 at 01:40:03PM +0200, Philipp Puschmann wrote:
> Hi Uwe
> 
> Am 19.09.19 um 13:22 schrieb Uwe Kleine-König:
> > On Thu, Sep 19, 2019 at 12:26:28PM +0200, Philipp Puschmann wrote:
> >> Using only 4 DMA periods for UART RX is very few if we have a high
> >> frequency of small transfers - like in our case using Bluetooth with
> >> many small packets via UART - causing many dma transfers but in each
> >> only filling a fraction of a single buffer. Such a case may lead to
> >> the situation that DMA RX transfer is triggered but no free buffer is
> >> available. While we have addressed the dma handling already with
> >> "dmaengine: imx-sdma: fix dma freezes" we still want to avoid
> > 
> > Is this statement still true now that you split this patch out of your
> > bigger series?
> Yes. The dma patches care about stopping DMA channel. This patch tries to
> avoid that the channel runs out of usable buffers (aka dma periods).
> 
> > 
> >> UART RX FIFO overrun. So we decrease the size of the buffers and
> >> increase their number and the total buffer size.
> > 
> > What happens when such an RX FIFO overrun happens? Are characters lost?
> > Or only time?
> Good question. In explanation i have missed an important point:
> When using HW flowcontrol via RTS/CTS and the buffer is full CTS is used to
> tell the remote device - here the Bluetooth chip - to stop sending data.
> For a while this prevents losing of characters. But then the remote device
> comes into trouble as its internal TX buffers runs over. Depends on the
> device how it handles this case and if it recovers if data flow is enabled
> again.
> 
> In case without HW flow control characters would be lost. Depends on the upper
> layer what happens then.
> 
> > Does your change have an influence if I do fewer but
> > bigger transfers?
> Don't think so. The dma periods are raw data buffers. If one is full the next one
> is being used. For the performance i don't see a significant difference between
> using 1 kB buffers or 4 kB buffers.

Would be great to add these infos to the commit log.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |