[U-Boot] [PATCH 3/3] sunxi: display: Add overscan correction

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 12-08-15 12:45, Ian Campbell wrote:
> On Wed, 2015-08-05 at 17:17 +0200, Hans de Goede wrote:
>>   > 	> sunxi_display.fb_size =
>>   > 	> 	> (mode->xres * mode->yres * 4 + 0xfff) & ~0xfff;
>> +> 	> overscan_offset = (overscan_y * mode->xres + overscan_x) * 4;
>> +> 	> /* We want to keep the fb_base for simplefb page aligned, where as
>> +> 	>  * the sunxi dma engines will happily accept an unaligned address. */
>> +> 	> if (overscan_offset)
>> +> 	> 	> sunxi_display.fb_size += 0x1000;
>
> Why plus 4K regardless of the magnitude of overscan_offset? What if it was
> 0x1004?

If overscan offset is 0x1004 then we make the simplefb start point to
fb_base + 0x2000:

sunxi_display.fb_addr = gd->fb_base
sunxi_display.fb_addr += (overscan_offset + 0xfff) & ~0xfff

And make the dma engine start at fb_base + 0xffc:

fb_dma_addr = gd->fb_base
fb_dma_addr += 0x1000 - (overscan_offset & 0xfff)

Notice how we make the dma_engine start at less then 0x1000 offset of
fb_base, so making fb_size 0x1000 larger is enough.

Basically to page-align the start of the simplefb / sunxi_display.fb_addr
we only need a single page, the rest we can get by offsetting
sunxi_display.fb_addr from fb_base by a multiple of the page size.

> Also, what's the link between fb_base's alignment and fb_size which is
> implied by the comment?

To align we need a page extra size as the dma-engine may start at
an offset of up-to a page-size from gd->fb_base and the dma-engine
will dma a full framebuffer size, including the black borders.

>
>>   > 	> gd->fb_base = gd->bd->bi_dram[0].start +
>>   > 	> 	>       gd->bd->bi_dram[0].size - sunxi_display.fb_size;
>>   > 	> sunxi_engines_init();
>> -> 	> sunxi_mode_set(mode, gd->fb_base - CONFIG_SYS_SDRAM_BASE);
>> +
>> +> 	> fb_dma_addr = gd->fb_base - CONFIG_SYS_SDRAM_BASE;
>> +> 	> sunxi_display.fb_addr = gd->fb_base;
>> +> 	> if (overscan_offset) {
>> +> 	> 	> fb_dma_addr += 0x1000 - (overscan_offset & 0xfff);
>> +> 	> 	> sunxi_display.fb_addr += (overscan_offset + 0xfff) & ~0xfff;
>> +> 	> 	> memset((void *)gd->fb_base, 0, sunxi_display.fb_size);
>> +> 	> 	> flush_cache(gd->fb_base, sunxi_display.fb_size);
>> +> 	> }
>
> Hrm, I think this starts to answer, but I'm still not sure I follow, sorry.

Correct this is the answer, sorry for the tricky maths, this is the best
solution without wasting a ton of memory with large overscans.

I hope the above explanation helps, if not keep asking.

>> +	sunxi_mode_set(mode, fb_dma_addr);
>>
>>   	/*
>>   > 	>  * These are the only members of this structure that are used. All the
>>   > 	>  * others are driver specific. There is nothing to decribe pitch or
>
>
> Pre-existing typo "describe".

Will fix,

Regards,

Hans