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

[Jon Hunter]

On 26/10/2022 05:44, Akhil R wrote:
> > 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@xxxxxxxxxx>
> > > Reviewed-by: Jon Hunter <jonathanh@xxxxxxxxxx>
> > > ---
> > >   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?

Unfortunately no. Yes for an old DTB without the dma-channel-mask 
property, we set the channel mask to 0xfffffffe, but this is not correct 
because it only has 31 interrupts/channels and not 32. So I think we 
will need to use of_irq_count() here.

Jon
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