[PATCH 1/2] fsl-layerscape: add dtb overlay feature

Sahil Malhotra (OSS) sahil.malhotra at oss.nxp.com
Wed Dec 8 07:12:17 CET 2021

Hi Michael,

-----Original Message-----
From: Michael Walle <michael at walle.cc> 
Sent: Monday, November 29, 2021 11:17 PM
To: Sahil Malhotra (OSS) <sahil.malhotra at oss.nxp.com>
Cc: Clément Faure <clement.faure at nxp.com>; Gaurav Jain <gaurav.jain at nxp.com>; Pankaj Gupta <pankaj.gupta at nxp.com>; Priyanka Jain <priyanka.jain at nxp.com>; u-boot at lists.denx.de; Varun Sethi <V.Sethi at nxp.com>; Ye Li <ye.li at nxp.com>; ZHIZHIKIN Andrey <andrey.zhizhikin at leica-geosystems.com>
Subject: Re: [PATCH 1/2] fsl-layerscape: add dtb overlay feature

Hi Sahil,

Am 2021-11-29 12:55, schrieb Sahil Malhotra (OSS):
> Am 2021-11-17 19:11, schrieb Sahil Malhotra (OSS):
>>> Could you please add some description what this is doing and for 
>>> what this is intended? To have a "DTB overlay feature", it is enough 
>>> to just enable CONFIG_OF_LIBFDT_OVERLAY.
>> I will add some description, and yes for DTB overlay feature, it is 
>> enough to enable CONFIG_OF_LIBFDT_OVERLAY but we need to do this step 
>> before booting the kernel that's why also have to enable 
>> Ok. What will the overlay do? Could you give an example?
> This overlay will be disabling the crypto nodes which will be used by 
> optee in secure world, so that linux should not use it.
>>> Apparently you're adding an overlay passed by optee. Doesn't this 
>>> have to be applied to u-boot's control dtb too?
>> Yes, we will be applying the overlay passed by optee, yes it will be 
>> applied to dtb which will be passed to uboot for kernel booting.
>> If I read this patch correctly, you're modifying the DT before you 
>> jump to linux. But I was asking whether you also have to modify the 
>> DT which is used by u-boot. Eg. if you disable some kind of crypto 
>> nodes (because optee will use them in secure world), this also have 
>> to communicated to u-boot, not only linux, no?
> Yes, I got your point now, and is very valid, but as of now for u-boot 
> we are just using the first available node for communicating with CAAM 
> leaving other job rings as it is.
> So we need not to apply overlay to DTB used by uboot.

> But we should do the correct thing, so that u-boot and linux doesn't see a different version of the device tree.

> Also what do you mean with "the first available node"?
> There is already a new CAAM driver for u-boot pending, see https://lore.kernel.org/u-boot/20211115070014.17586-1-gaurav.jain@nxp.com/

Very first CAAM Job Ring is always used by u-boot, OP-TEE does not use this job ring. Same job ring can be used by Linux once it boots up.

Sahil Malhotra

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