Re: [PATCH 08/12] linux-user: Add support for setting alsa timer enhanced read using ioctl

[Arnd Bergmann]

On Fri, Jan 17, 2020 at 9:50 PM Aleksandar Markovic
<address@hidden> wrote:

> Alexandre (and Arnd too, or any other person knowledgeable in the area),
>
> I just need to clarify a couple of details with you, please.
>
> Firstly, here is what man page rtc(4) says:
>
> "The /dev/rtc (or /dev/rtc0, /dev/rtc1, etc.) device can be opened
> only once (until it is closed) and it is read-only. On read(2) and
> select(2) the calling process is blocked until the next interrupt from
> that RTC is received. Following the interrupt, the process can read a
> long integer, of which the least significant byte contains a bit mask
> encoding the types of interrupt that occurred, while the remaining 3
> bytes contain the number of interrupts since the last read(2)."
>
> So, it looks read() will always return only 4 bytes of useful info
> (regardless of host being 32-bit/64-bit).

It says "long integer", which is 64-bit on a 64-bit machine.

> My questions are:
>
> - Is the description in man page genuinely accurate?

Starting with linux-2.6.18, there is another possibility: If an
application asks for exactly four bytes on a 64-bit kernel,
it gets the lower four bytes, as it would on a 32-bit kernel.

This is a hack that was introduced for running 32-bit compat
tasks.

For any other size less than sizeof(long), the kernel reports
an EINVAL error, and for anything larger or equal to sizeof(long)
it attempts to output a long word.

> - To me (but I am really an outsider to using RTC in applications),
> this feature (blocking read()/select()) even looks very nice and
> convenient, in all fairness. But I would like to ask you: Is this
> feature used rarely or frequently by other libraries/tools/etc.? In
> other words, is the feature "obscure" or "crucial" part of RTC kernel
> support? Or, something in between?

> - Does MC146818 support this feature?

No idea, I'll leave these for Alexandre or someone else to answer.

      Arnd