linux-kernel - RE: [PATCH v2 3/3] dmaengine: tegra: Add support for dma-channel-mask

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Date:   Fri, 23 Sep 2022 11:09:07 +0000
From:   Akhil R <akhilrajeev@...dia.com>
To:     Jonathan Hunter <jonathanh@...dia.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 3/3] dmaengine: tegra: Add support for dma-channel-mask

> On 23/09/2022 11:17, Akhil R wrote:
> >> On 19/09/2022 12:25, Akhil R wrote:
> >>> Add support for dma-channel-mask so that only the specified channels
> >>> are used. This helps to reserve some channels for the firmware.
> >>>
> >>> This was initially achieved by limiting the channel number to 31 in
> >>> the driver and adjusting the register address to skip channel0 which
> >>> was reserved for a firmware. Now, with this change, the driver can
> >>> align more to the actual hardware which has 32 channels.
> >>>
> >>> Signed-off-by: Akhil R <akhilrajeev@...dia.com>
> >>> ---
> >>>    drivers/dma/tegra186-gpc-dma.c | 37 +++++++++++++++++++++++++++---
> ----
> >>>    1 file changed, 30 insertions(+), 7 deletions(-)
> >>>
> >>> diff --git a/drivers/dma/tegra186-gpc-dma.c b/drivers/dma/tegra186-gpc-
> >> dma.c
> >>> index fa9bda4a2bc6..1d1180db6d4e 100644
> >>> --- a/drivers/dma/tegra186-gpc-dma.c
> >>> +++ b/drivers/dma/tegra186-gpc-dma.c
> >>> @@ -161,7 +161,10 @@
> >>>    #define TEGRA_GPCDMA_BURST_COMPLETION_TIMEOUT	5000 /* 5
> >> msec */
> >>>
> >>>    /* Channel base address offset from GPCDMA base address */
> >>> -#define TEGRA_GPCDMA_CHANNEL_BASE_ADD_OFFSET	0x20000
> >>> +#define TEGRA_GPCDMA_CHANNEL_BASE_ADDR_OFFSET	0x10000
> >>
> >> Why did this value change? There is no mention in the commit message. If
> >> this was incorrect before, then this needs to be a separate patch and
> >> tagged with the appropriate fixes tag so that this can be picked up for
> >> stable.
> > This is mentioned in the commit message.
> >
> > "... and adjusting the register address to skip channel0 ..."
> >
> > Probably it is not very clear that it directs to this change. Shall I update the
> > commit message to have a clearer context?
> 
> Ah OK. I was wondering how this worked with 'channel_reg_size' but
> looking closer I see channel_reg_size is always SZ_64K. I wonder why we
> even bother having this parameter and don't use SZ_64K directly?
There is an offset from the base address which the per channel registers start.
Although this offset value happens to match with the channel_reg_size, this is
not actually the per channel register size.
> 
> Anyway, for now this is fine.
> 
Thanks for the review.

Regards,
Akhil