[U-Boot] [PATCH v2 5/6] sunxi: add code for recalculating the DRAM size in U-Boot

[Maxime Ripard]

On Thu, Mar 29, 2018 at 01:21:38PM +0100, Andre Przywara wrote:
> Hi,
> 
> On 29/03/18 10:37, Maxime Ripard wrote:
> > On Wed, Mar 28, 2018 at 07:31:51PM +0800, Icenowy Zheng wrote:
> >> 于 2018年3月28日 GMT+08:00 下午7:28:07, Maxime Ripard <maxime.ripard at bootlin.com> 写到:
> >>> On Mon, Mar 26, 2018 at 03:11:04PM +0800, Icenowy Zheng wrote:
> >>>>
> >>>>
> >>>> 于 2018年3月26日 GMT+08:00 下午3:06:33, Maxime Ripard
> >>> <maxime.ripard at bootlin.com> 写到:
> >>>>> On Fri, Mar 23, 2018 at 05:41:43PM +0800, Icenowy Zheng wrote:
> >>>>>>
> >>>>>>
> >>>>>> 于 2018年3月23日 GMT+08:00 下午5:40:41, Maxime Ripard
> >>>>> <maxime.ripard at bootlin.com> 写到:
> >>>>>>> On Fri, Mar 23, 2018 at 04:18:56PM +0800, Icenowy Zheng wrote:
> >>>>>>>> The get_ram_size() function in U-Boot can only deal with memory
> >>>>> size
> >>>>>>>> smaller than 2GiB. To enable the support of 3GiB DRAM on newer
> >>>>> 64-bit
> >>>>>>>> SoCs, an alternative way to detect DRAM size is needed.
> >>>>>>>
> >>>>>>> Why not just fixing get_ram_size then?
> >>>>>>
> >>>>>> Even if it's fixed it won't support 3GiB DRAM at all.
> >>>>>
> >>>>> Why?
> >>>>
> >>>> It has an assumption that the size is pow of 2.
> >>>
> >>> I guess this would be fixable too? (or one could create a variant
> >>> without that assumption).
> >>
> >> I don't think its principle allows such kind of fix, as it just
> >> checks writing then reading at some offset that is pow if 2.
> > 
> > You could do have a bunch of algorithm actually. One would be to write
> > the address in memory and try to detect where exactly it starts to
> > loop.
> > 
> > You could do a bisection in the opposite direction once you settled
> > for the upper limit (so you would have for example a workable 2G, a
> > non-workable 4G, and then you try intervals that you always divide by
> > two, so testing then 3G (that works), then halfway between 3G and 4G,
> > etc.
> > 
> >> For hacking it, see my implementation in v1, which assumes the
> >> only size supported bigger than 2GiB is 3GiB (which is
> >> acceptable on sunxi, but might not work on other platforms).
> >>
> >> As Andre said, that function has another big problem -- it detects
> >> memory with writing to it. This is risky.
> > 
> > How is it risky when it's done by the SPL?
> 
> Originally that was my confusion as well: It's not the SPL calling that
> function. The DRAM controller init function in there knows very
> precisely how much DRAM we have, but we don't communicate this to U-Boot
> proper. So U-Boot *proper* goes ahead and probes the DRAM. This means it
> could step into secure memory, for instance. On sunxi64 we have the ATF
> running between SPL and U-Boot, also all kind of secure payloads could
> already have been registered.
> So I wonder if it would be easier to somehow pass on this *one* word of
> information between SPL and U-Boot proper to avoid calling this function
> altogether?

That would definitely make sense yes.

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com
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