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

[Jon Hunter <jonathanh@nvidia.com>]

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@nvidia.com>
>>> Reviewed-by: Jon Hunter <jonathanh@nvidia.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?

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
-- 
nvpublic