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

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Date:   Thu, 19 Sep 2019 13:22:16 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Philipp Puschmann <philipp.puschmann@...ix.com>
Cc:     linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        yibin.gong@....com, fugang.duan@....com, l.stach@...gutronix.de,
        jslaby@...e.com, shawnguo@...nel.org, s.hauer@...gutronix.de,
        kernel@...gutronix.de, festevam@...il.com, linux-imx@....com,
        linux-serial@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2] serial: imx: adapt rx buffer and dma periods

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?

> 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? Does your change have an influence if I do fewer but
bigger transfers?

Best regards
Uwe

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