Re: [PATCH v8 04/10] hw/ssi: imx_spi: Rework imx_spi_reset() to keep CONREG register value

[Peter Maydell]

On Thu, 28 Jan 2021 at 14:18, Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>
> On 1/28/21 2:54 PM, Peter Maydell wrote:
> > On Thu, 28 Jan 2021 at 13:47, Bin Meng <bmeng.cn@gmail.com> wrote:
> >>
> >> On Thu, Jan 28, 2021 at 9:43 PM Peter Maydell <peter.maydell@linaro.org> 
> >> wrote:
> >>> This retains the CONREG register value for both:
> >>>  * 'soft' reset caused by write to device register to disable
> >>>    the block
> >>>    -- this is corrcet as per the datasheet quote
> >>>  * 'power on' reset via TYPE_DEVICE's reset method
> >>>    -- but in this case we should reset CONREG, because the Device
> >>>    reset method is like a complete device powercycle and should
> >>>    return the device state to what it was when QEMU was first
> >>>    started.
> >>
> >> The POR value of CONREG is zero, which should be the default value, no?
> >
> > But you're not setting it to zero here, you're leaving it with
> > whatever value it had before. (That's correct for soft reset,
> > but wrong for power-on.)
>
> zero value on power-on is what I tried to describe as
> "It is initialized to zero when the instance is created."

Yes, but QOM device reset does not happen just once at startup and
not thereafter. Consider:

 * user starts QEMU
 * QOM devices are created and realized
 * QOM device reset happens
     -- CONREG is zero here because QOM structs are zero-initialized
 * guest runs
 * guest modifies CONREG from its initial value
 * system reset is requested (perhaps by user, perhaps by
   guest writing some register or another)
 * QOM device reset happens
     -- CONREG is not zero here, so reset must clear it

thanks
-- PMM