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 <[email protected]> > 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 > ><[email protected]> 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 > >> ><[email protected]> 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? Maxime -- Maxime Ripard, Bootlin (formerly Free Electrons) Embedded Linux and Kernel engineering https://bootlin.com
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