Re: [RFC 2.6.26-rc9 1/5] pxafb: add shared framebuffer interface

["Jaya Kumar" <jayakumar.lkml@xxxxxxxxx>]

On Mon, Jul 21, 2008 at 8:32 AM, Jaya Kumar <jayakumar.lkml@xxxxxxxxx> wrote:
> On Mon, Jul 21, 2008 at 7:11 AM, Russell King - ARM Linux
> <linux@xxxxxxxxxxxxxxxx> wrote:
>> On Sun, Jul 20, 2008 at 02:10:29AM -0400, Jaya Kumar wrote:
>>
>>> @@ -1357,6 +1379,8 @@ static void pxafb_decode_mode_info(struct pxafb_info *fbi,
>>>               smemlen = modes[i].xres * modes[i].yres * modes[i].bpp / 8;
>>>               if (smemlen > fbi->fb.fix.smem_len)
>>>                       fbi->fb.fix.smem_len = smemlen;
>>> +             if (inf->custom_xfer_div)
>>> +                     fbi->fb.fix.smem_len /= inf->custom_xfer_div;
>>
>> I don't understand this.  If you have a 640x480 display with 8 bits of
>> memory per pixel, then it's 307200 bytes no matter whether you're only
>> using the lower 4 bits and the upper 4 are unused.
>>
>> If, on the other hand, the lower 4 bits contain one pixel and the upper
>> 4 bits contain the other pixel, then you have a 4 bit per pixel
>> framebuffer, so 'bpp' should be 4.  In which case the original calculation
>> gives the correct answer.
>>
>
> The complexity I faced is that the transfers used by Metronome on the
> AMLCD bus have to be 16-bits wide although it is actually treating the
> pixels as 8-bits per pixel and then only using the lower 4-bits within
> that for actual content. So what I did in the code is set bits per
> pixel, bpp to 16 so that pxafb sets the LCCR3_16BPP flag (so that it
> meets Metronome's AMLCD transfer requirements) and then use the
> custom_xfer_div to divide the transfer sizes and buffer allocations to
> the correct size.
>
> If I set bpp to 8, the allocations and transfer sizes are correct but
> then that causes pxafb to set LCCR3_8BPP and that causes Metronome to
> fail to display anything.
>
> Instead of doing custom_xfer_div, I had thought about setting
> pxafb_mach_info.lccr3 = LCCR3_16BPP to try to force the setting of
> LCCR3_16BPP but then I saw:
>
> include/asm/arch-pxa/pxafb.h
>         * The following should not be defined in LCCR3
>         *      LCCR3_HSP, LCCR3_VSP, LCCR0_Pcd(x), LCCR3_Bpp
>         */
>        u_int           lccr3;
>
> so I decided to create custom_xfer_div instead. How about if I try a
> different way like creating a separate flag that can be used to
> predicate the lccr3 setting. Maybe something like:
>
> static int pxafb_bpp_to_lccr3(struct fb_var_screeninfo *var)
> ...
>        case 8:
>               switch (inf->custom_amlcd_bpp) {
>                case 16: ret = LCCR3_16BPP; break;
>                default: ret = LCCR3_8BPP; break;
>        case 16: ret = LCCR3_16BPP; break;
>
> Thanks,
> jaya
>

I took a look at doing the above, ie: adjusting bpp instead of
custom_xfer_size. I've realized that I would still need to adjust lccr
in at least one other place:

        fbi->reg_lccr1 =
                LCCR1_DisWdth(var->xres) +

I'll try to improve my explanation. :-)

The reason for this is that metronome requires a different bus width
(16 bit) for AMLCD transfers than it requires for its expected bits
per pixel ( 8 bits). To get LCDC to use 16 bits on the AMLCD bus, we
need to set 16bpp in LCCR3. So we then need to divide down the x
resolution in LCCR1. Eg: 800 pixels at 8bpp is the framebuffer. We
need 16-bit transfers so we set the desired pixels per line in LCCR to
= 800 pixels / ( 16 custom bus bits / 8 bits per pixel ) = 400 pixels
per line. So the code would be like:

pix_per_line = var->xres / ( inf->custom_lccr_bpp / var->bits_per_pixel )

I hope I was able to explain my reasoning for this above.

Thanks,
jaya

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