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

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Date:   Thu, 9 Mar 2023 15:36:44 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Oliver Neukum <oneukum@...e.com>,
        Ye Xiang <xiang.ye@...el.com>, Arnd Bergmann <arnd@...db.de>,
        Matthias Kaehlcke <mka@...omium.org>,
        Lee Jones <lee@...nel.org>, Wolfram Sang <wsa@...nel.org>,
        Tyrone Ting <kfting@...oton.com>,
        Mark Brown <broonie@...nel.org>,
        Bartosz Golaszewski <brgl@...ev.pl>, linux-usb@...r.kernel.org,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-spi@...r.kernel.org, linux-gpio@...r.kernel.org,
        srinivas.pandruvada@...el.com, heikki.krogerus@...ux.intel.com,
        sakari.ailus@...ux.intel.com, zhifeng.wang@...el.com,
        wentong.wu@...el.com, lixu.zhang@...el.com
Subject: Re: [PATCH v4 2/5] gpio: Add support for Intel LJCA USB GPIO driver

On Thu, Mar 09, 2023 at 03:18:53PM +0100, Linus Walleij wrote:
> On Thu, Mar 9, 2023 at 2:53 PM Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> 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.

To make it all simple, just make obuf and ibuf pointers to the data you
allocate with a call to kmalloc().

thanks,

greg k-h