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

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Date:   Wed, 26 Oct 2022 04:44:03 +0000
From:   Akhil R <akhilrajeev@...dia.com>
To:     Rob Herring <robh@...nel.org>
CC:     Laxman Dewangan <ldewangan@...dia.com>,
        Jonathan Hunter <jonathanh@...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>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>
Subject: RE: [PATCH RESEND v2 3/3] dmaengine: tegra: Add support for
 dma-channel-mask

> On Thu, Oct 20, 2022 at 02:03:22PM +0530, 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>
> > Reviewed-by: Jon Hunter <jonathanh@...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
> > +
> > +/* Default channel mask reserving channel0 */
> > +#define TEGRA_GPCDMA_DEFAULT_CHANNEL_MASK    0xfffffffe
> >
> >  struct tegra_dma;
> >  struct tegra_dma_channel;
> > @@ -246,6 +249,7 @@ struct tegra_dma {
> >       const struct tegra_dma_chip_data *chip_data;
> >       unsigned long sid_m2d_reserved;
> >       unsigned long sid_d2m_reserved;
> > +     u32 chan_mask;
> >       void __iomem *base_addr;
> >       struct device *dev;
> >       struct dma_device dma_dev;
> > @@ -1288,7 +1292,7 @@ static struct dma_chan *tegra_dma_of_xlate(struct
> of_phandle_args *dma_spec,
> >  }
> >
> >  static const struct tegra_dma_chip_data tegra186_dma_chip_data = {
> > -     .nr_channels = 31,
> > +     .nr_channels = 32,
> 
> This is an ABI break. A new kernel with an old DTB will use 32 channels
> instead of 31. You should leave this and use the dma-channel-mask to
> enable all 32 channels.
> 
Hi Rob,

If using an old DTB, tdma->chan_mask will be default to 0xfffffffe since it
would not have the dma-channel-mask property. The driver would still 
use 31 channels even if it uses an old DTB. Shouldn't it prevent the
ABI break? 

Regards,
Akhil