Date:	Tue, 21 Dec 2010 12:22:46 +1300
From:	Ryan Mallon <ryan@...ewatersys.com>
To:	Alexey Charkov <alchark@...il.com>
CC:	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org,
	vt8500-wm8505-linux-kernel@...glegroups.com,
	Eric Miao <eric.y.miao@...il.com>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Albin Tonnerre <albin.tonnerre@...e-electrons.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6 v9] ARM: Add basic architecture support for VIA/WonderMedia
 85xx SoC's

On 12/21/2010 12:00 PM, Alexey Charkov wrote:
> 2010/12/21 Ryan Mallon <ryan@...ewatersys.com>:
>> On 12/21/2010 10:48 AM, Alexey Charkov wrote:
>>> 2010/12/20 Ryan Mallon <ryan@...ewatersys.com>:
>>>> On 12/21/2010 08:54 AM, Alexey Charkov wrote:
>>>>> This adds support for the family of Systems-on-Chip produced initially
>>>>> by VIA and now its subsidiary WonderMedia that have recently become
>>>>> widespread in lower-end Chinese ARM-based tablets and netbooks.
>>>>>
>>>>> Support is included for both VT8500 and WM8505, selectable by a
>>>>> configuration switch at kernel build time.
>>>>>
>>>>> Included are basic machine initialization files, register and
>>>>> interrupt definitions, support for the on-chip interrupt controller,
>>>>> high-precision OS timer, GPIO lines, necessary macros for early debug,
>>>>> pulse-width-modulated outputs control, as well as platform device
>>>>> configurations for the specific drivers implemented elsewhere.
>>>>>
>>>>> Signed-off-by: Alexey Charkov <alchark@...il.com>
>>>>
>>>> Hi Alexey,
>>>>
>>>> Quick review below.
>>
>>> <snip>
>>>>> +void __init wmt_set_resources(void)
>>>>> +{
>>>>> +     resources_lcdc[0].start = wmt_current_regs->lcdc;
>>>>> +     resources_lcdc[0].end = wmt_current_regs->lcdc + SZ_1K - 1;
>>>>> +     resources_lcdc[1].start = wmt_current_irqs->lcdc;
>>>>> +     resources_lcdc[1].end = wmt_current_irqs->lcdc;
>>>>
>>>> Ah, this makes more sense. But why have all the indirection? The
>>>> wmt_regmaps table could just be replaced with #defines and then have
>>>> separate device files for the VT8500 and the WM8505. This would also
>>>> make clearer which variants have which peripherals.
>>>>
>>>
>>> This was the way I implemented it originally. However, Arnd made quite
>>> a valid suggestion to allow runtime selection of the chip variant,
>>> thus registers and interrupts need to be held in an indexed data type
>>> instead of just compile-time macros. In addition, there is now some
>>> overall movement towards unification of binary kernel images for
>>> different ARM variants (as far as I can see), so this would be
>>> required in any case.
>>>
>>> Furthermore, as with many unbranded Chinese products, it's somewhat
>>> difficult to reliably determine the exact chip version used in your
>>> netbook without disassembling it. Reading a hardware register for
>>> identification is easier :)
>>
>> Okay, that makes sense. I still think there must be a better way than
>> having a massive indirect table with all the values. Why not detect the
>> variant in the core code and then have something like:
>>
>> int init_devices(void)
>> {
>>        int board_type = detect_board_type();
>>
>>        switch (board_type) {
>>        case BOARD_TYPE_VT8500:
>>                return vt8500_init_devices();
>>
>>        case BOARD_TYPE_WM8505:
>>                return wm8500_init_devices();
>>        }
>>
>>        pr_err("Unknown board type\n");
>>        BUG(); /* panic()? */
>>        while (1)
>>                ;
>> }
>>
>> Then you can have the peripheral setup for each of the variants in their
>> own files and use #defines. It may get tricky in a couple of places if
>> you need to be able to access some value directly which is different on
>> each of the variants, but that can be handled on a case by case basis.
>> The majority of the numbers will be passed into drivers via the resource
>> structs.
>>
> 
> This is more or less what I'm doing right now - except for the
> separation between different files. I tried to avoid duplication of
> similar things here. Is the indirect table really a big issue? I'm a
> bit reluctant to copy about the whole devices.c for each chip variant,
> which would be otherwise required. Further, it would add more
> complexity to the timer, irq, gpio, i8042 and probably some other
> places.

Yeah, agreed about the duplication. My approach would require the common
peripherals to be defined for each variant. I'm not entirely against the
indirect table (and am even starting to think it may be the best overall
solution), it's just that it can be a bit difficult to follow because to
add a device you need to do the following:

 - Add a partially empty resource/platform_device struct
 - Add resource entries to the resource table definition
 - Add resource values to the resource table
 - Add assignment of resource values to device init code

The indirection also makes it harder to quickly determine the IRQ number
of memory address of a peripheral.

The current solution using the indirect table is okay, but it would be
nice to find a solution which reduces some of this effort.

>>>>> +
>>>>> +void __init vt8500_reserve_mem(void)
>>>>> +{
>>>>> +#ifdef CONFIG_FB_VT8500
>>>>> +     panels[current_panel_idx].bpp = 16; /* Always use RGB565 */
>>>>> +     preallocate_fb(&panels[current_panel_idx], SZ_4M);
>>>>> +     vt8500_device_lcdc.dev.platform_data = &panels[current_panel_idx];
>>>>> +#endif
>>>>> +}
>>>>
>>>> Not sure if this should exist in the platform code or the framebuffer
>>>> driver. In the latter case it would automatically be CONFIG_FB_VT8500
>>>> and the platform_data can still be set in the platform code. Is there a
>>>> reason for this not to be in the framebuffer driver?
>>>>
>>>
>>> I can't reserve memory via memblock from the driver, and usual runtime
>>> allocation functions can't handle it (need alignment to 4 megabytes in
>>> 8500, framebuffer sizes exceed 4 megabytes in 8505).
>>
>> Can you use one of the initcalls from a driver to to the memblock
>> reserve? I don't know much about how memblock works. There are also the
>> various large page allocators in the works, but I don't think anything
>> has hit mainline yet.
>>
> 
> No, drivers are initialized too late, even if compiled statically.
> Memblock can only be reserved through the designated machine callback,
> as far as I have understood.

Okay.

>>>>> +}
>>>>> +
>>>>> +int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns)
>>>>> +{
>>>>> +     unsigned long long c;
>>>>> +     unsigned long period_cycles, prescale, pv, dc;
>>>>> +
>>>>> +     if (pwm == NULL || period_ns == 0 || duty_ns > period_ns)
>>>>> +             return -EINVAL;
>>>>> +
>>>>> +     c = 25000000/2; /* wild guess --- need to implement clocks */
>>>>> +     c = c * period_ns;
>>>>> +     do_div(c, 1000000000);
>>>>> +     period_cycles = c;
>>>>
>>>> This looks like it could be reworked to remove the do_div call.
>>>>
>>>
>>> I just followed PXA implementation in this regard. Are there any
>>> specific suggestions? Note that c should not be a complie-time
>>> constant eventually, as this is the guessed PWM base frequency (should
>>> be read from the hardware, but the code for clocks is not yet in).
>>
>> I didn't have a particular solution in mind, but often by changing the
>> units used and rearranging the math a bit you can often avoid having to
>> do horrible multiplies and divides.
>>
>> For now at least you could avoid the do_div by assigning period_cycles
>> directly.
>>
> 
> It depends on period_ns, which is passed in as an argument from
> whatever uses PWM, so I'm not sure it can be assigned directly.

Ah. How big a number is period_ns? Can you do something like this instead?

	period_cycles = ((250 / 2) * period_ns) / 10000;

and still safely avoid overflows?

~Ryan
	
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