Re: [PATCH v4 2/5] gpio: Add support for Intel LJCA USB GPIO driver

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

On Thu, Mar 9, 2023 at 2:53 PM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> On Thu, Mar 09, 2023 at 02:40:10PM +0100, Oliver Neukum wrote:
> > On 09.03.23 08:10, Ye Xiang wrote:
> >
> > > +#define LJCA_GPIO_BUF_SIZE 60
> > > +struct ljca_gpio_dev {
> > > +   struct platform_device *pdev;
> > > +   struct gpio_chip gc;
> > > +   struct ljca_gpio_info *gpio_info;
> > > +   DECLARE_BITMAP(unmasked_irqs, LJCA_MAX_GPIO_NUM);
> > > +   DECLARE_BITMAP(enabled_irqs, LJCA_MAX_GPIO_NUM);
> > > +   DECLARE_BITMAP(reenable_irqs, LJCA_MAX_GPIO_NUM);
> > > +   u8 *connect_mode;
> > > +   /* mutex to protect irq bus */
> > > +   struct mutex irq_lock;
> > > +   struct work_struct work;
> > > +   /* lock to protect package transfer to Hardware */
> > > +   struct mutex trans_lock;
> > > +
> > > +   u8 obuf[LJCA_GPIO_BUF_SIZE];
> > > +   u8 ibuf[LJCA_GPIO_BUF_SIZE];
> >
> > And here we have a violation of DMA coherency rules.
> > Basically you cannot embed buffers into other data structures
> > if they can be subject to DMA.
>
> Huh?!
>
> The problem here is alignment. But other than that I can't see the issue with
> embedding into structures the instances of which will be allocated on the heap.

Yups. And I think the solution looks something like this:

u8 obuf[LJCA_GPIO_BUF_SIZE] __aligned(8);
u8 ibuf[LJCA_GPIO_BUF_SIZE] __aligned(8);

__aligned(4) if it's 32bit DMA I guess? 8 always works that's
why we use it all over the IIO subsystem.

Yours,
Linus Walleij