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SubjectRe: [PATCH v4 06/11] drm/sun4i: add support for Allwinner DE2 mixers

于 2017年4月20日 GMT+08:00 下午4:37:07, Maxime Ripard
<maxime.ripard@free-electrons.com> 写到:
> On Tue, Apr 18, 2017 at 06:47:56PM +0800, Icenowy Zheng wrote:
>> >> + /* Get the physical address of the buffer in memory */
>> >> + gem = drm_fb_cma_get_gem_obj(fb, 0);
>> >> +
>> >> + DRM_DEBUG_DRIVER("Using GEM @ %pad\n", &gem->paddr);
>> >> +
>> >> + /* Compute the start of the displayed memory */
>> >> + bpp = fb->format->cpp[0];
>> >> + paddr = gem->paddr + fb->offsets[0];
>> >> + paddr += (state->src_x >> 16) * bpp;
>> >> + paddr += (state->src_y >> 16) * fb->pitches[0];
>> >> +
>> >> + DRM_DEBUG_DRIVER("Setting buffer address to %pad\n", &paddr);
>> >> +
>> >> + paddr_u32 = (uint32_t) paddr;
>> >
>> >How does that work on 64-bits systems ?
>>
>> The hardware is not designed to work on 64-bit systems.
>>
>> Even 64-bit A64/H5 has also 3GiB memory limit.
>
> That's a fragile assumption.

Yes, it's only the basical reason.

>
>> The address cell in mixer hardware is also only 32-bit.
>>
>> So we should just keep the force conversion here. If we then really
>> met 4GiB-capable AW SoC without changing DE2, I think we should have
>> other way to limit CMA pool inside 4GiB.
>
> The register name looks like this is only the lower 32 bits that you
> can set here, and that there is another register for the upper 32 bits
> of that address somewhere.

Maybe... but no one can verify this as their's no currently any user
which has 4GiB+ DRAM.

I think we should keep this until Allwinner really made a 4GiB-capable
hardware.

>
> In that case, please use the lower_32_bits and upper_32_bits helper,
> and don't cast it that way.
>
> If it isn't the case, you should set the DMA mask (through
> dma_set_mask) so that we only allocate memory that can be accessed by
> this device.

How to do it?

>
> Maxime

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