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Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [Patch v7 1/8] memory: tegra: add interconnect support for DRAM scaling in Tegra234 To: Krzysztof Kozlowski , , , , , , , , CC: , , , , , , , , , , , , , Sumit Gupta References: <20230424131337.20151-1-sumitg@nvidia.com> <20230424131337.20151-2-sumitg@nvidia.com> <7c6c6584-204a-ada1-d669-2e8bef50e5e5@linaro.org> <3071273b-b03b-5fc8-ffa1-9b18311a3a5d@nvidia.com> <5ab9687e-756d-f94b-b085-931d4ea534c1@nvidia.com> <10b32e55-4d28-5405-035e-c73a514c95e4@linaro.org> <14438cf9-ec78-afb5-107a-4ed954ac0eb7@nvidia.com> <0815c0b5-304b-568f-5a64-d19d7d2aeb93@linaro.org> Content-Language: en-US From: Sumit Gupta In-Reply-To: <0815c0b5-304b-568f-5a64-d19d7d2aeb93@linaro.org> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.126.231.37] X-ClientProxiedBy: rnnvmail202.nvidia.com (10.129.68.7) To rnnvmail201.nvidia.com (10.129.68.8) X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BN8NAM11FT029:EE_|CH2PR12MB4070:EE_ X-MS-Office365-Filtering-Correlation-Id: d5d73416-18af-4439-8d07-08db509c6169 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 09 May 2023 14:47:58.0609 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: d5d73416-18af-4439-8d07-08db509c6169 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.160];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: BN8NAM11FT029.eop-nam11.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH2PR12MB4070 Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org On 09/05/23 19:06, Krzysztof Kozlowski wrote: > External email: Use caution opening links or attachments > > > On 09/05/2023 15:17, Sumit Gupta wrote: >>>>>>> + /* >>>>>>> + * MC driver probe can't get BPMP reference as >>>>>>> it gets probed >>>>>>> + * earlier than BPMP. So, save the BPMP ref got >>>>>>> from the EMC >>>>>>> + * DT node in the mc->bpmp and use it in MC's >>>>>>> icc_set hook. >>>>>>> + */ >>>>>>> + mc->bpmp = emc->bpmp; >>>>>> >>>>>> This (and ()) are called without any locking. You register first the >>>>>> interconnect, so set() callback can be used, right? Then set() could be >>>>>> called anytime between tegra_emc_interconnect_init() and assignment >>>>>> above. How do you synchronize these? >>>>>> >>>>>> Best regards, >>>>>> Krzysztof >>>>>> >>>>> >>>>> Currently, the tegra234_mc_icc_set() has NULL check. So, it will give >>>>> this error. >>>>> if (!mc->bpmp) { >>> >>> How does it solve concurrent accesses and re-ordering of instructions by >>> compiler or CPU? >>> >> >> Now, the "mc->bpmp" is set before tegra_emc_interconnect_init(). >> So, until the EMC interconnect initializes, set() won't be >> called as the devm_of_icc_get() call will fail. > > What if compiler puts "mc->bpmp" assignment after > tegra_emc_interconnect_init()? > > What if CPU executes above assignment also after > tegra_emc_interconnect_init()? > > Considering amount of code inside tegra_emc_interconnect_init() second > case is rather unlikely, but first possible, right? > > Best regards, > Krzysztof > Yes, for protection from compiler re-ordering I can add barrier() after the assignment. mc->bpmp = emc->bpmp; barrier(); Thank you, Sumit Gupta