Re: [PATCH v5 1/2] gpio: add GPO driver for PCA9570

[Sungbo Eo]

Thanks, it made me think about it deeper...

On 20. 7. 8. ìì 12:07, Andy Shevchenko wrote:
> On Tue, Jul 7, 2020 at 5:03 PM Sungbo Eo <mans0n@xxxxxxxxxx> wrote:
> > On 20. 7. 6. ìí 9:00, Andy Shevchenko wrote:
>
> ...
>
> > But I don't really understand what mutex does here. The driver does not
> > need consecutive commands, it only sends/receives only one byte at a
> > time. And AFAIK each i2c_smbus function is already protected by a mutex.
> > So what should be exactly inside the lock? Should we protect the output
> > buffer as well? I'm not an expert on this so please enlighten me.
>
> There are questions, answering them will give you a solution:
> - Since we have two functions doing i2c communications, can they
> clash? If so, does the i2c framework guarantee the serialisation?

I think it does.

> - Since we have a shared resource (buf), can accessors clash? How do
> we guarantee serialization?

But the output buffer should be tied to the i2c operations. So I guess 
it requires a mutex here.

pca9570_get() does not access gpio->out so it does not need to be locked.

On the other hand, the whole pca9570_set() function should be protected, 
from reading gpio->out to rewriting to gpio->out. So pca9570_write() 
error check should be inside the lock as well. Am I right?

Thanks.