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SubjectRe: [PATCH 01/14] dmaengine: dma-jz4780: Avoid hardcoding number of channels


Le sam. 7 juil. 2018 à 9:34, PrasannaKumar Muralidharan
<prasannatsmkumar@gmail.com> a écrit :
> On 5 July 2018 at 23:56, Paul Cercueil <paul@crapouillou.net> wrote:
>> Hi PrasannaKumar,
>>
>>
>>> Hi Paul,
>>>
>>> On 3 July 2018 at 18:02, Paul Cercueil <paul@crapouillou.net>
>>> wrote:
>>>>
>>>> As part of the work to support various other Ingenic JZ47xx SoC
>>>> versions,
>>>> which don't feature the same number of DMA channels per core, we
>>>> now
>>>> deduce the number of DMA channels available from the devicetree
>>>> compatible string.
>>>>
>>>> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
>>>> ---
>>>> drivers/dma/dma-jz4780.c | 53
>>>> +++++++++++++++++++++++++++++-----------
>>>> 1 file changed, 39 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/drivers/dma/dma-jz4780.c b/drivers/dma/dma-jz4780.c
>>>> index 85820a2d69d4..b40f491f0367 100644
>>>> --- a/drivers/dma/dma-jz4780.c
>>>> +++ b/drivers/dma/dma-jz4780.c
>>>> @@ -16,6 +16,7 @@
>>>> #include <linux/interrupt.h>
>>>> #include <linux/module.h>
>>>> #include <linux/of.h>
>>>> +#include <linux/of_device.h>
>>>> #include <linux/of_dma.h>
>>>> #include <linux/platform_device.h>
>>>> #include <linux/slab.h>
>>>> @@ -23,8 +24,6 @@
>>>> #include "dmaengine.h"
>>>> #include "virt-dma.h"
>>>>
>>>> -#define JZ_DMA_NR_CHANNELS 32
>>>> -
>>>> /* Global registers. */
>>>> #define JZ_DMA_REG_DMAC 0x1000
>>>> #define JZ_DMA_REG_DIRQP 0x1004
>>>> @@ -135,14 +134,20 @@ struct jz4780_dma_chan {
>>>> unsigned int curr_hwdesc;
>>>> };
>>>>
>>>> +enum jz_version {
>>>> + ID_JZ4780,
>>>> +};
>>>> +
>>>> struct jz4780_dma_dev {
>>>> struct dma_device dma_device;
>>>> void __iomem *base;
>>>> struct clk *clk;
>>>> unsigned int irq;
>>>> + unsigned int nb_channels;
>>>> + enum jz_version version;
>>>>
>>>> uint32_t chan_reserved;
>>>> - struct jz4780_dma_chan chan[JZ_DMA_NR_CHANNELS];
>>>> + struct jz4780_dma_chan chan[];
>>>
>>>
>>> Looks like a variable length array in struct. I think there is some
>>> effort to remove the usage of VLA. Can you revisit this? I may be
>>> wrong, please feel free to correct.
>>
>>
>> Are you sure? It's the first time I hear about it.
>> Could anybody confirm?
>
> Please see [1] for info.
>
> Variable Length Arrays in struct is expressly forbidden in C99, C11.
> Clang does not support it. To make kernel compile with Clang few
> people are trying to remove/reduce VLAIS usage.
>
> 1.
> https://blog.linuxplumbersconf.org/2013/ocw/system/presentations/1221/original/VLAIS.pdf

I read it, and my structure is not a VLAIS; my "chan" array is a
flexible
array, its sizeof() is 0, so the sizeof() of the structure is constant.

See page 6 of the PDF, about alternatives to VLAIS:
"If possible use a flexible array member and move the array to the end
of
the struct"
Which is what I am doing here.

-Paul

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