Re: [RFC PATCH 1/9] apple-gmux: use cpu_to_be32 instead of manual reorder

[Hans de Goede]

Hi,

On 2/11/23 00:30, Orlando Chamberlain wrote:
> On Fri, 10 Feb 2023 20:19:27 +0100
> Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
> 
>> Hi,
>>
>> On 2/10/23 20:09, Hans de Goede wrote:
>>> Hi,
>>>
>>> On 2/10/23 05:48, Orlando Chamberlain wrote:  
>>>> Currently it manually flips the byte order, but we can instead use
>>>> cpu_to_be32(val) for this.
>>>>
>>>> Signed-off-by: Orlando Chamberlain <orlandoch.dev@xxxxxxxxx>
>>>> ---
>>>>  drivers/platform/x86/apple-gmux.c | 18 ++----------------
>>>>  1 file changed, 2 insertions(+), 16 deletions(-)
>>>>
>>>> diff --git a/drivers/platform/x86/apple-gmux.c
>>>> b/drivers/platform/x86/apple-gmux.c index
>>>> 9333f82cfa8a..e8cb084cb81f 100644 ---
>>>> a/drivers/platform/x86/apple-gmux.c +++
>>>> b/drivers/platform/x86/apple-gmux.c @@ -94,13 +94,7 @@ static u32
>>>> gmux_pio_read32(struct apple_gmux_data *gmux_data, int port)
>>>> static void gmux_pio_write32(struct apple_gmux_data *gmux_data,
>>>> int port, u32 val) {
>>>> -	int i;
>>>> -	u8 tmpval;
>>>> -
>>>> -	for (i = 0; i < 4; i++) {
>>>> -		tmpval = (val >> (i * 8)) & 0xff;
>>>> -		outb(tmpval, gmux_data->iostart + port + i);
>>>> -	}
>>>> +	outl(cpu_to_be32(val), gmux_data->iostart + port);
>>>>  }
>>>>  
>>>>  static int gmux_index_wait_ready(struct apple_gmux_data
>>>> *gmux_data)  
>>>
>>> The ioport / indexed-ioport accessed apple_gmux-es likely are (part
>>> of?) LPC bus devices . Looking at the bus level you are now
>>> changing 4 io accesses with a size of 1 byte, to 1 32 bit io-access.
>>>
>>> Depending on the decoding hw in the chip this may work fine,
>>> or this may work not at all.
>>>
>>> I realized that you have asked for more testing, but most surviving
>>> macbooks from the older apple-gmux era appear to be models without
>>> a discrete GPU (which are often the first thing to break) and thus
>>> without a gmux.
>>>
>>> Unless we get a bunch of testers to show up, which I doubt. I would
>>> prefer slightly bigger / less pretty code and not change the
>>> functional behavior of the driver on these older models.  
>>
>> A quick follow up on this, I just noticed that only the pio_write32
>> is doing the one byte at a time thing:
>>
>> static u32 gmux_pio_read32(struct apple_gmux_data *gmux_data, int
>> port) {
>>         return inl(gmux_data->iostart + port);
>> }
>>
>> static void gmux_pio_write32(struct apple_gmux_data *gmux_data, int
>> port, u32 val)
>> {
>>         int i;
>>         u8 tmpval;
>>
>>         for (i = 0; i < 4; i++) {
>>                 tmpval = (val >> (i * 8)) & 0xff;
>>                 outb(tmpval, gmux_data->iostart + port + i);
>>         }
>> }
>>
>> And if you look closely gmux_pio_write32() is not swapping
>> the order to be32 at all, it is just taking the bytes
>> in little-endian memory order, starting with the first
>> (index 0) byte which is the least significant byte of
>> the value.
>>
>> On x86 the original code is no different then doing:
>>
>> static void gmux_pio_write32(struct apple_gmux_data *gmux_data, int
>> port, u32 val)
>> {
>>         u8 *data = (u8 *)&val;
>>         int i;
>>
>>         for (i = 0; i < 4; i++)
>>                 outb(data[i], gmux_data->iostart + port + i);
>> }
>>
>> So yeah this patch is definitely wrong, it actually swaps
>> the byte order compared to the original code. Which becomes
>> clear when you look the weird difference between the read32 and
>> write32 functions after this patch.
>>
>> Presumably there is a specific reason why gmux_pio_write32()
>> is not already doing a single outl(..., val) and byte-ordering
>> is not the reason.
>>
>> Regards,
>>
>> Hans
> 
> Sounds like it may be better to just drop this patch as there's very
> little benefit for the risk of causing a regression.

Yes if it is easy to drop this please drop this.

And the same more or less applies to 2/9. I would rather have
an extra "if () ... else ..."  in the code in a couple of places
then change behavior on old hw where we cannot get proper test
coverage (but will likely eventually get regressions reports
if we break things).

Thanks & Regards,

Hans



>>>> @@ -177,16 +171,8 @@ static u32 gmux_index_read32(struct
>>>> apple_gmux_data *gmux_data, int port) static void
>>>> gmux_index_write32(struct apple_gmux_data *gmux_data, int port,
>>>> u32 val) {
>>>> -	int i;
>>>> -	u8 tmpval;
>>>> -
>>>>  	mutex_lock(&gmux_data->index_lock);
>>>> -
>>>> -	for (i = 0; i < 4; i++) {
>>>> -		tmpval = (val >> (i * 8)) & 0xff;
>>>> -		outb(tmpval, gmux_data->iostart + GMUX_PORT_VALUE
>>>> + i);
>>>> -	}
>>>> -
>>>> +	outl(cpu_to_be32(val), gmux_data->iostart +
>>>> GMUX_PORT_VALUE); gmux_index_wait_ready(gmux_data);
>>>>  	outb(port & 0xff, gmux_data->iostart + GMUX_PORT_WRITE);
>>>>  	gmux_index_wait_complete(gmux_data);  
>>>   
>>
>