Hi Andrew,
On Wed, Jan 12, 2022 at 2:38 PM Andrew Lunn andrew@lunn.ch wrote:
If an optional IRQ is not present, drivers either just ignore it (e.g. for devices that can have multiple interrupts or a single muxed IRQ), or they have to resort to polling. For the latter, fall-back handling is needed elsewhere in the driver. To me it sounds much more logical for the driver to check if an optional irq is non-zero (available) or zero (not available), than to sprinkle around checks for -ENXIO. In addition, you have to remember that this one returns -ENXIO, while other APIs use -ENOENT or -ENOSYS (or some other error code) to indicate absence. I thought not having to care about the actual error code was the main reason behind the introduction of the *_optional() APIs.
The *_optional() functions return an error code if there has been a real error which should be reported up the call stack. This excludes whatever error code indicates the requested resource does not exist, which can be -ENODEV etc. If the device does not exist, a magic cookie is returned which appears to be a valid resources but in fact is not. So the users of these functions just need to check for an error code, and fail the probe if present.
Agreed.
Note that in most (all?) other cases, the return type is a pointer (e.g. to struct clk), and NULL is the magic cookie.
You seems to be suggesting in binary return value: non-zero (available) or zero (not available)
Only in case of success. In case of a real failure, an error code must be returned.
This discards the error code when something goes wrong. That is useful information to have, so we should not be discarding it.
No, the error code must be retained in case of failure.
IRQ don't currently have a magic cookie value. One option would be to add such a magic cookie to the subsystem. Otherwise, since 0 is invalid, return 0 to indicate the IRQ does not exist.
Exactly. And using 0 means the similar code can be used as for other subsystems, where NULL would be returned.
The only remaining difference is the "dummy cookie can be passed to other functions" behavior. Which is IMHO a valid difference, as unlike with e.g. clk_prepare_enable(), you do pass extra data to request_irq(), and sometimes you do need to handle the absence of the interrupt using e.g. polling.
The request for a script checking this then makes sense. However, i don't know how well coccinelle/sparse can track values across function calls. They probably can check for:
ret = irq_get_optional() if (ret < 0) return ret;
A missing if < 0 statement somewhere later is very likely to be an error. A comparison of <= 0 is also likely to be an error. A check for
0 before calling any other IRQ functions would be good. I'm
surprised such a check does not already existing in the IRQ API, but there are probably historical reasons for that.
There are still a few platforms where IRQ 0 does exist.
Gr{oetje,eeting}s,
Geert
-- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds