[PATCH] drm/ast: Fix ARM compatibility

[Jammy Huang]

Hi Thomas,

The Intel(x86) CPUs have a separate address space for "IO", but the ARM 
architecture only has "memory", so all IO devices are accessed as if 
they were memory. Which means ARM does not support isolated IO. Here is 
a related discussion on ARM's forum.

https://community.arm.com/support-forums/f/architectures-and-processors-forum/52046/how-to-read-write-an-i-o-port-in-aarch64

Thus, we want to adapt MMIO only after this patch.

On 2023/4/17 下午 07:51, Thomas Zimmermann wrote:
> Hi
>
> Am 07.04.23 um 04:09 schrieb Jammy Huang:
>> Hi Thomas,
>>
>> Could you help review this patch??
>>
>> We met some problem on nvidia's ARM platfrom and need this patch to 
>> fix it.
>>
>> On 2023/3/2 上午 10:19, Jammy Huang wrote:
>>> ARM architecture only has 'memory', so all devices are accessed by 
>>> MMIO.
>>>
>>> Signed-off-by: Jammy Huang <jammy_huang at aspeedtech.com>
>>> ---
>>>   drivers/gpu/drm/ast/ast_main.c | 17 +----------------
>>>   1 file changed, 1 insertion(+), 16 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/ast/ast_main.c 
>>> b/drivers/gpu/drm/ast/ast_main.c
>>> index 794ffd4a29c5..f86d01e9f024 100644
>>> --- a/drivers/gpu/drm/ast/ast_main.c
>>> +++ b/drivers/gpu/drm/ast/ast_main.c
>>> @@ -424,22 +424,7 @@ struct ast_device *ast_device_create(const 
>>> struct drm_driver *drv,
>>>       if (!ast->regs)
>>>           return ERR_PTR(-EIO);
>>> -    /*
>>> -     * If we don't have IO space at all, use MMIO now and
>>> -     * assume the chip has MMIO enabled by default (rev 0x20
>>> -     * and higher).
>>> -     */
>>> -    if (!(pci_resource_flags(pdev, 2) & IORESOURCE_IO)) {
>>> -        drm_info(dev, "platform has no IO space, trying MMIO\n");
>>> -        ast->ioregs = ast->regs + AST_IO_MM_OFFSET;
>>> -    }
>>> -
>>> -    /* "map" IO regs if the above hasn't done so already */
>>> -    if (!ast->ioregs) {
>>> -        ast->ioregs = pcim_iomap(pdev, 2, 0);
>
> What happens on systems that use this branch?
>
> Best regards
> Thomas
>
>>> -        if (!ast->ioregs)
>>> -            return ERR_PTR(-EIO);
>>> -    }
>>> +    ast->ioregs = ast->regs + AST_IO_MM_OFFSET;
>>>       ast_detect_chip(dev, &need_post);
>>>
>>> base-commit: 254986e324add8a30d0019c6da59f81adc8b565f
>>
>
-- 
Best Regards
Jammy
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