linux-kernel - Re: [PATCH v3 4/5] dmaengine: xilinx_dma: autodetect whether the HW supports scatter-gather

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Date:   Fri, 29 Jun 2018 09:53:11 +0200
From:   Andrea Merello <andrea.merello@...il.com>
To:     Vinod <vkoul@...nel.org>
Cc:     dan.j.williams@...el.com, michal.simek@...inx.com,
        appana.durga.rao@...inx.com, dmaengine@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree <devicetree@...r.kernel.org>,
        Radhey Shyam Pandey <radhey.shyam.pandey@...inx.com>
Subject: Re: [PATCH v3 4/5] dmaengine: xilinx_dma: autodetect whether the HW
 supports scatter-gather

On Fri, Jun 29, 2018 at 9:37 AM, Vinod <vkoul@...nel.org> wrote:
> On 25-06-18, 11:27, Andrea Merello wrote:
>> The AXIDMA and CDMA HW can be either direct-access or scatter-gather
>> version. These are SW incompatible.
>>
>> The driver can handle both version: a DT property was used to
>                                 ^^^^
> versions
>

OK

>> tell the driver whether to assume the HW is is scatter-gather mode.
>                                            ^^^^^
> is in?

Yes, it is. Thanks

>>
>> This patch makes the driver to autodetect this information. The DT
>> property is not required anymore.
>>
>> No changes for VDMA.
>>
>> Cc: Rob Herring <robh+dt@...nel.org>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: devicetree@...r.kernel.org
>> Cc: Radhey Shyam Pandey <radhey.shyam.pandey@...inx.com>
>> Signed-off-by: Andrea Merello <andrea.merello@...il.com>
>> Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@...inx.com>
>> ---
>> Changes in v2:
>>       - autodetect only in !VDMA case
>> Changes in v3:
>>       - cc DT maintainers/ML
>> ---
>>  drivers/dma/xilinx/xilinx_dma.c | 14 ++++++++++----
>>  1 file changed, 10 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/dma/xilinx/xilinx_dma.c b/drivers/dma/xilinx/xilinx_dma.c
>> index 7f0ab904b749..43fcc71ff287 100644
>> --- a/drivers/dma/xilinx/xilinx_dma.c
>> +++ b/drivers/dma/xilinx/xilinx_dma.c
>> @@ -86,6 +86,7 @@
>>  #define XILINX_DMA_DMASR_DMA_DEC_ERR         BIT(6)
>>  #define XILINX_DMA_DMASR_DMA_SLAVE_ERR               BIT(5)
>>  #define XILINX_DMA_DMASR_DMA_INT_ERR         BIT(4)
>> +#define XILINX_DMA_DMASR_SG_MASK             BIT(3)
>>  #define XILINX_DMA_DMASR_IDLE                        BIT(1)
>>  #define XILINX_DMA_DMASR_HALTED              BIT(0)
>>  #define XILINX_DMA_DMASR_DELAY_MASK          GENMASK(31, 24)
>> @@ -406,7 +407,6 @@ struct xilinx_dma_config {
>>   * @dev: Device Structure
>>   * @common: DMA device structure
>>   * @chan: Driver specific DMA channel
>> - * @has_sg: Specifies whether Scatter-Gather is present or not
>>   * @mcdma: Specifies whether Multi-Channel is present or not
>>   * @flush_on_fsync: Flush on frame sync
>>   * @ext_addr: Indicates 64 bit addressing is supported by dma device
>> @@ -426,7 +426,6 @@ struct xilinx_dma_device {
>>       struct device *dev;
>>       struct dma_device common;
>>       struct xilinx_dma_chan *chan[XILINX_DMA_MAX_CHANS_PER_DEVICE];
>> -     bool has_sg;
>>       bool mcdma;
>>       u32 flush_on_fsync;
>>       bool ext_addr;
>> @@ -2393,7 +2392,6 @@ static int xilinx_dma_chan_probe(struct xilinx_dma_device *xdev,
>>
>>       chan->dev = xdev->dev;
>>       chan->xdev = xdev;
>> -     chan->has_sg = xdev->has_sg;
>>       chan->desc_pendingcount = 0x0;
>>       chan->ext_addr = xdev->ext_addr;
>>       /* This variable ensures that descriptors are not
>> @@ -2486,6 +2484,15 @@ static int xilinx_dma_chan_probe(struct xilinx_dma_device *xdev,
>>               chan->stop_transfer = xilinx_dma_stop_transfer;
>>       }
>>
>> +     /* check if SG is enabled (only for AXIDMA and CDMA) */
>> +     if (xdev->dma_config->dmatype != XDMA_TYPE_VDMA) {
>> +             if (dma_ctrl_read(chan, XILINX_DMA_REG_DMASR) &
>> +                 XILINX_DMA_DMASR_SG_MASK)
>
> why not read this for VDMA too, will it return false?

AFAIK this bit is reserved in VDMA, so reading it can lead to any
random thing.. I would say that potentially it could even be used for
other purposes in future IP releases..

>> +                     chan->has_sg = true;
>> +             dev_dbg(chan->dev, "ch %d: SG %s\n", chan->id,
>> +                     chan->has_sg ? "enabled" : "disabled");
>
> this debug print can be removed

IMHO if someone will ever enable debug prints for debugging something
and then he/she reports us a log, then it would be useful to see in
the log if the IP was SG or not..

> --
> ~Vinod