[Mesa-dev] [PATCH] egl/android: rework device probing

[Tomasz Figa]

Hi Emil,

On Fri, Aug 24, 2018 at 9:25 PM Emil Velikov <emil.l.velikov at gmail.com> wrote:
>
> From: Emil Velikov <emil.velikov at collabora.com>
>
> Unlike the other platforms, here we aim do guess if the device that we
> somewhat arbitrarily picked, is supported or not.
>
> In particular: when a vendor is _not_ requested we loop through all
> devices, picking the first one which can create a DRI screen.
>
> When a vendor is requested - we use that and do _not_ fall-back to any
> other device.
>
> The former seems a bit fiddly, but considering EGL_EXT_explicit_device and
> EGL_MESA_query_renderer are MIA, this is the best we can do for the
> moment.
>
> With those (proposed) extensions userspace will be able to create a
> separate EGL display for each device, query device details and make the
> conscious decision which one to use.
>
> Cc: Robert Foss <robert.foss at collabora.com>
> Cc: Tomasz Figa <tfiga at chromium.org>
> Signed-off-by: Emil Velikov <emil.velikov at collabora.com>
> ---
> Thanks for the clarification Tomasz. The original code was using a
> fall-back even a vendor was explicitly requested, confusing me a bit ;-)

Yeah, it was surprisingly difficult to get it right and I missed that
in the review. Thanks for figuring this out.

> ---
>  src/egl/drivers/dri2/platform_android.c | 71 +++++++++++++++----------
>  1 file changed, 43 insertions(+), 28 deletions(-)
>
> diff --git a/src/egl/drivers/dri2/platform_android.c b/src/egl/drivers/dri2/platform_android.c
> index 1f9fe27ab85..5bf627dec7d 100644
> --- a/src/egl/drivers/dri2/platform_android.c
> +++ b/src/egl/drivers/dri2/platform_android.c
> @@ -1420,13 +1420,32 @@ droid_filter_device(_EGLDisplay *disp, int fd, const char *vendor)
>     return 0;
>  }
>
> +static int
> +droid_probe_device(_EGLDisplay *disp)
> +{
> +  /* Check that the device is supported, by attempting to:
> +   * - load the dri module
> +   * - and, create a screen
> +   */
> +   if (!droid_load_driver(disp)) {
> +      _eglLog(_EGL_WARNING, "DRI2: failed to load driver");

Failing to create a screen is probably critical enough to be worth a
message, even if just probing, but failing to load a driver would like
mean that there is no driver for the device, so perhaps we don't need
to clutter the log in such case?

> +      return -1;
> +   }
> +
> +   if (!dri2_create_screen(disp)) {
> +      _eglLog(_EGL_WARNING, "DRI2: failed to create screen");

I don't remember if I got an answer to this question, sorry if I did:

Is there really nothing to clean up after droid_load_driver()? Not
leaking anything here, when probing though the subsequent devices?

> +      return -1;
> +   }
> +   return 0;
> +}
> +
>  static int
>  droid_open_device(_EGLDisplay *disp)
>  {
>  #define MAX_DRM_DEVICES 32
>     drmDevicePtr device, devices[MAX_DRM_DEVICES] = { NULL };
>     int prop_set, num_devices;
> -   int fd = -1, fallback_fd = -1;
> +   int fd = -1;
>
>     char *vendor_name = NULL;
>     char vendor_buf[PROPERTY_VALUE_MAX];
> @@ -1451,33 +1470,39 @@ droid_open_device(_EGLDisplay *disp)
>           continue;
>        }
>
> -      if (vendor_name && droid_filter_device(disp, fd, vendor_name)) {
> -         /* Match requested, but not found - set as fallback */
> -         if (fallback_fd == -1) {
> -            fallback_fd = fd;
> -         } else {
> +      /* If a vendor is explicitly provided, we use only that.
> +       * Otherwise we fall-back the first device that is supported.
> +       */
> +      if (vendor_name) {
> +         if (droid_filter_device(disp, fd, vendor_name)) {
> +            /* Device does not match - try next device */
>              close(fd);
>              fd = -1;
> +            continue;
>           }
> -
> +         /* If the requested device matches use it, regardless if
> +          * init fails. Do not fall-back to any other device.
> +          */
> +         if (droid_probbe_device(disp)) {

typo: droid_probe_device

> +            close(fd);
> +            fd = -1;
> +         }
> +         break;
> +      }
> +      /* No explicit request - attempt the next device */
> +      if (droid_probbe_device(disp)) {

typo: droid_probe_device

> +         close(fd);
> +         fd = -1;
>           continue;
>        }
> -      /* Found a device */
>        break;

A break at the end of a loop is really confusing to read. Could we
rewrite the last block to avoid it? E.g.

   if (!droid_probe_device(disp)) {
      /* Found a device */
      break;
   }
   /* No explicit request - attempt the next device. */
   close(fd);
   fd = -1;
}

Best regards,
Tomasz