Messages in this thread | | | From | Prashant Malani <> | Date | Wed, 22 Sep 2021 11:47:57 -0700 | Subject | Re: [PATCH] platform/chrome: cros_ec_proto: Fix check_features ret val |
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Hi Enric,
On Wed, Sep 22, 2021 at 7:09 AM Enric Balletbo i Serra <enric.balletbo@collabora.com> wrote: > > Hi Prashant, > > On 22/9/21 12:55, Prashant Malani wrote: > > Hi Enric, > > > > On Wed, Sep 22, 2021 at 2:12 AM Enric Balletbo i Serra > > <enric.balletbo@collabora.com> wrote: > >> > >> Hi Prashant, > >> > >> On 21/9/21 19:29, Prashant Malani wrote: > >>> Hi Enric, > >>> > >>> Thanks for reviewing the patch. > >>> > >>> On Tue, Sep 21, 2021 at 01:42:04PM +0200, Enric Balletbo i Serra wrote: > >>>> Hi Prashant, > >>>> > >>>> Thank you for the patch. Just one comment below ... > >>>> > >>>> On 16/9/21 3:46, Prashant Malani wrote: > >>>>> The kerneldoc for cros_ec_check_features() states that it returns 1 or 0 > >>>>> depedending on whether a feature is supported or not, but it instead > >>>>> returns a negative error number in one case, and a non-1 bitmask in > >>>>> other cases. > >>>>> > >>>>> Since all call-sites only check for a 1 or 0 return value, update > >>>>> the function to return boolean values. > >>>>> > >>>>> Signed-off-by: Prashant Malani <pmalani@chromium.org> > >>>>> --- > >>>>> drivers/platform/chrome/cros_ec_proto.c | 12 +++++++----- > >>>>> include/linux/platform_data/cros_ec_proto.h | 2 +- > >>>>> 2 files changed, 8 insertions(+), 6 deletions(-) > >>>>> > >>>>> diff --git a/drivers/platform/chrome/cros_ec_proto.c b/drivers/platform/chrome/cros_ec_proto.c > >>>>> index a7404d69b2d3..a34cf58c5ef7 100644 > >>>>> --- a/drivers/platform/chrome/cros_ec_proto.c > >>>>> +++ b/drivers/platform/chrome/cros_ec_proto.c > >>>>> @@ -808,9 +808,9 @@ EXPORT_SYMBOL(cros_ec_get_host_event); > >>>>> * > >>>>> * Call this function to test whether the ChromeOS EC supports a feature. > >>>>> * > >>>>> - * Return: 1 if supported, 0 if not > >>>>> + * Return: true if supported, false if not (or if an error was encountered). > >>>>> */ > >>>>> -int cros_ec_check_features(struct cros_ec_dev *ec, int feature) > >>>>> +bool cros_ec_check_features(struct cros_ec_dev *ec, int feature) > >>>>> { > >>>>> struct cros_ec_command *msg; > >>>>> int ret; > >>>>> @@ -818,8 +818,10 @@ int cros_ec_check_features(struct cros_ec_dev *ec, int feature) > >>>>> if (ec->features[0] == -1U && ec->features[1] == -1U) { > >>>>> /* features bitmap not read yet */ > >>>>> msg = kzalloc(sizeof(*msg) + sizeof(ec->features), GFP_KERNEL); > >>>>> - if (!msg) > >>>>> - return -ENOMEM; > >>>>> + if (!msg) { > >>>>> + dev_err(ec->dev, "failed to allocate memory to get EC features\n"); > >>>> > >>>> In case of failure you will be noticed by the allocator, prints after > >>>> [k|v][m|z|c]alloc() functions are not needed, so I think you can just return > >>>> false here. > >>>>> > >>> > >>> Makes sense; I can make the change, but I had one question: > >>> > >>> If we solely return false, how will we tell from the logs that the > >>> allocation error message was associated with this driver? Only returning > >>> false means the driver probe (e.g cros-ec-typec) will continue (just assuming a certain feature > >>> is not present). Wouldn't having this error message make this clear? > >>> > >> > >> So I tried to find some doc about this without luck. But I think it has been an > >> unwritten rule that GFP_KERNEL allocations for small allocations will never > >> fail. > > > > That might be the case, but kzalloc() still returns the error value, > > so even if it is very unlikely, we > > still need to handle that error. > > > > I'm not saying that we don't need to handle that error, we *must* handle the > allocation errors, there is no doubts about this, but in this specific function > we're ignoring all the errors and not noticing to the users. The discussion, or > my nitpick (it's just a nit), is about if logging that the allocation failed in > this driver is useful or not. > > > >> If you system fails to allocate that small amount of memory you probably > >> have bigger problems to solve and the above message is not really useful, even > >> confusing, as the focus, likely, shouldn't be in this driver to solve the problem. > > > > I don't know if I necessarily concur with that rationale ("if it > > fails, there are likely bigger issues"). > > There could be situations (hypothetical) where a series of allocations > > might lead to a failure (or this might be a transient allocation > > failure), > > and it might be useful to know which driver is contributing the alloc > > that finally precipitates the failure. > > > > If the kernel is not able to give you a small amount of memory it is just a > coincidence that this happened in your driver. Dynamic allocations and > deallocations happens often, the print doesn't gives you more information as it > could be any other driver because your system runs out of memory, and everybody > can be affected. > > > Also, although it is very unlikely, returning true without an error > > I'm not saying this, I'm saying returning false but don't log the error to the > console. > > > can mean the typec driver silently continues to function > > I think the consensus on the interface is, and was clear: > > 1/True if the feature is supported, 0/false if is not supported or there is an > error. At some point we decided that the callers don't need to differentiate > about if the feature is not supported or there is and unlikely error. >
I wasn't present when this was agreed on, but I don't think that's the right approach; moreover the current implementation doesn't adhere to it. In general I prefer either returning the appropriate error code, or parsing the error and logging it.
> > wrongly assuming a feature flag to be set a certain way. That is > > something we need to flag through the logs. > > If you run out of memory there will be a lot of logs, trust me ;-) > > > I certainly can't see the log message as confusing the reader of a log > > any further. > > > > As I said there is an unwritten rule (didn't find written doc about it tbh, > although I know other maintainers follow) that prints after [k|v][m|z|c]alloc() > functions are not needed.
Right, but in most cases they return -ENOMEM; here the boolean return type prevents that.
> > And as I said, this is a nit, I won't strongly oppose if you think that this > message is really needed.
I'd prefer sticking with this if it's possible; in any case we'll have a follow up patch soon which gets rid of the kzalloc entirely (as you suggested) so this discussion will become moot.
Best regards,
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