linux-kernel - Re: [PATCH] dmaengine: ti: omap-dma: Configure LCH

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Date:   Sun, 25 Nov 2018 11:57:08 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Tony Lindgren <tony@...mide.com>
Cc:     Aaro Koskinen <aaro.koskinen@....fi>,
        Peter Ujfalusi <peter.ujfalusi@...com>, vkoul@...nel.org,
        dan.j.williams@...el.com, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org
Subject: Re: [PATCH] dmaengine: ti: omap-dma: Configure LCH_TYPE for OMAP1

On Sun, Nov 25, 2018 at 11:11:05AM +0000, Russell King - ARM Linux wrote:
> On Sat, Nov 24, 2018 at 05:07:17PM -0800, Tony Lindgren wrote:
> > * Russell King - ARM Linux <linux@...linux.org.uk> [181124 20:10]:
> > > On Fri, Nov 23, 2018 at 08:52:15PM +0200, Aaro Koskinen wrote:
> > > > Hi,
> > > > 
> > > > On Fri, Nov 23, 2018 at 02:35:04PM +0200, Peter Ujfalusi wrote:
> > > > > On 22/11/2018 17.12, Russell King - ARM Linux wrote:
> > > > > > I'm also not sure about this:
> > > > > > 
> > > > > >         if (cpu_is_omap15xx())
> > > > > >                 end++;
> > > > > > 
> > > > > > in dma_dest_len() - is that missing from the omap-dma driver?  It looks
> > > > > > like a work-around for some problem on OMAP15xx, but I can't make sense
> > > > > > about why it's in the UDC driver rather than the legacy DMA driver.
> > > > > 
> > > > > afaik no other legacy drivers were doing similar thing, this must be
> > > > > something which is needed for the omap_udc driver to fix up something?
> > > > 
> > > > Here's the patch that added it: https://marc.info/?l=linux-omap&m=119634396324221&w=2
> > > > 
> > > > "Make DMA-OUT behave on the 1510 ... the 1510 CPC register was just
> > > > off-by-one with respect to the 1611 CDAC"
> > > 
> > > ... which suggests that's a problem with the CPC register itself, and
> > > we should fix that in the DMAengine driver rather than the USB gadget
> > > driver.
> > > 
> > > Tony, any input on this?
> > 
> > Yeah that sounds like some hardware work-around for 15xx as described
> > in the DMA_DEST_LAST macro reading CSAC on 15xx instead of CDAC. Seems
> > like it should be done in the dmaengine driver.. My guess is that other
> > dma users never needed to read CSAC register?
> 
> Hmm, reading the OMAP1510 TRM for the CPC register, it seems that
> omap-dma won't handle this particularly well.  The fact that the
> register only updates after the last request in an element or frame
> means that if we try to use this value as the current source /
> destination address before the first transfer, it can be wildly
> wrong.
> 
> Saving the current value at the beginning of a request, and detecting
> if it's changed (like omap_udc) isn't going to work well in the
> generic case.  If, say, the register happens to contain 0x0004, and
> our next transfer is using 32-bit elements in element sync mode
> starting at address with lsb 16 bits as 0, it would mean we'd see
> 0x0004 in this register after the _second_ element has been
> transferred, and we'd assume that the register hasn't yet changed -
> but we would have in reality transferred two elements.
> 
> However, omap-dma.c zeros the CPC register before each transfer,
> which is interesting, because in one place the OMAP1510 TRM says
> that the CPC register is read/write, but in the actual description
> of this register, it says it's read-only.
> 
> What it also means is that, in such a case, after the 2nd element has
> been transferred, where the register contains 0x0004, the address
> we'd be looking for (to calculate the residual) should be 0x0008,
> not 0x0005, so we actually need to be adding the number of bytes
> according to element size.
> 
> Looking at omap-dma.c, someone has added support for the residue
> granularity:
> 
>         if (__dma_omap15xx(od->plat->dma_attr))
>                 od->ddev.residue_granularity =
>                                 DMA_RESIDUE_GRANULARITY_DESCRIPTOR;
>         else
>                 od->ddev.residue_granularity = DMA_RESIDUE_GRANULARITY_BURST;

I'll point out that this was introduced by:

commit c9bd0946da243a8eb86b44ff613e2c813f9b683b
Author: Janusz Krzysztofik <jmkrzyszt@...il.com>
Date:   Tue Jun 5 18:59:57 2018 +0200

    dmaengine: ti: omap-dma: Fix OMAP1510 incorrect residue_granularity
...
    It was verified already before that omap_get_dma_src_pos() from
    arch/arm/plat-omap/dma.c didn't work correctly for OMAP1510 - see
    commit 1bdd7419910c ("ASoC: OMAP: fix OMAP1510 broken PCM pointer
    callback") for details.  Apparently the same applies to its successor,
    omap_dma_get_src_pos() from drivers/dma/ti/omap-dma.c.

So, this now presents us with bigger problems - if we fix it now for
omap_udc, should the above commit be reverted, and if we do revert
the above commit, will it regress OMAP1510 audio.

The intention of omap-dma was always to report an accurate residue.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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