Re: Looking for issues/features for my first contribution

["Alex Bennée" <alex.bennee@linaro.org>]

Rajath Shashidhara <rajaths@cs.utexas.edu> writes:

> On 07-11-2019 07:33, Aleksandar Markovic wrote:
>> I did a quick Google search on datasheets of existing RTC
>> implemtations, and the result is:
>> DS1338:
>> https://datasheets.maximintegrated.com/en/ds/DS1338-DS1338Z.pdf
>> M41T80: https://www.st.com/resource/en/datasheet/m41t80.pdf
>> M48T59: http://www.elektronikjk.pl/elementy_czynne/IC/M48T59V.pdf
>> MC146818: https://www.nxp.com/docs/en/data-sheet/MC146818.pdf
>> PL031: http://infocenter.arm.com/help/topic/com.arm.doc.ddi0224c/real_time_clock_pl031_r1p3_technical_reference_manual_DDI0224C.pdf
>> TWL92230: https://datasheet.octopart.com/TWL92230C-Texas-Instruments-datasheet-150321.pdf
>> Zynq RTC: https://www.xilinx.com/support/documentation/user_guides/ug1085-zynq-ultrascale-trm.pdf
>> (chapter 7)
>
> I have a few questions about this:
> [a] Is there any particular reason that you picked DS3231 ? Linux
> kernel has drivers for DS3232/34 only [1]. I did read the datasheets
> of both 3232 & 3231 and found that they are quite similar except for
> the 236 bytes of SRAM support found only in 3232.
>
> [b] As per the datasheet, DS3231 has a built-in temperature sensor.
> Temperature can be read from a dedicated register. There can be two
> approaches to emulating this: (1) Return a constant temperature value
> on every read (2) Throw a not-supported exception/warning. What is the
> qemu convention for handling such features ?

Don't throw an exception. You can at the minimum do a
qemu_log_mask(LOG_UNIMP) to indicate the system is using currently
unimplemented functionality. Alternatively wire-up a device property via
QOM so the user can vary the reported temperature.

QEMU currently doesn't have a decent API for exposing values for dynamic
emulated sensors to the outside world aside from QMP for chaning device
values. It's something we have discussed in the past but the trick is
coming up with something that can cover the wide range of device types.
Maybe QMP is good enough?

>
> [c] DS3231 also has programmable square-wave output + 32 KHz output
> pin. M41T80 chip also supports this feature. However, qemu does not
> support emulation of these features [2]. Do I take the same approach ?
>
> Thanks!
> Rajath Shashidhara
>
> References:
> [1]
> https://elixir.bootlin.com/linux/v5.4-rc6/source/drivers/rtc/rtc-ds3232.c
> [2]
> https://git.qemu.org/?p=qemu.git;a=blob;f=hw/rtc/m41t80.c;h=914ecac8f4db418633d6daf92608cb50f6b89052;hb=HEAD


--
Alex Bennée