linux-kernel - [PATCH] cpufreq: tegra186: Use flexible array to simplify memory allocation

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Message-Id: <f6b75a33df6f5fd94da3cfecb1e9e7590bf8cd37.1668963937.git.christophe.jaillet@wanadoo.fr>
Date:   Sun, 20 Nov 2022 18:19:02 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>
Cc:     linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        linux-pm@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: [PATCH] cpufreq: tegra186: Use flexible array to simplify memory allocation

Use flexible array to simplify memory allocation.
It saves some memory, avoids an indirection when reading the 'clusters'
array and removes some LoC.


Detailed explanation:
====================
Knowing that:
  - each devm_ allocation over-allocates 40 bytes for internal needs
  - Some rounding is done by the memory allocator on 8, 16, 32, 64, 96,
    128, 192, 256, 512, 1024, 2048, 4096, 8192 boundaries

and that:
  - sizeof(struct tegra186_cpufreq_data) = 24
  - sizeof(struct tegra186_cpufreq_cluster) = 16

Memory allocations in tegra186_cpufreq_probe() are:
  data:           (24 + 40) = 64 		      => 64 bytes
  data->clusters: (2 * 16 + 40) = 72     => 96 bytes
So a total of 160 bytes are allocated.
56 for the real need, 80 for internal uses and 24 are wasted.


If 'struct tegra186_cpufreq_data' is reordered so that 'clusters' is a
flexible array:
  - it saves one pointer in the structure
  - only one allocation is needed

So, only 96 bytes are allocated:
  16 + 2 * 16 + 40 = 88  => 96 bytes

Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
---
Compile tested only
---
 drivers/cpufreq/tegra186-cpufreq.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/cpufreq/tegra186-cpufreq.c b/drivers/cpufreq/tegra186-cpufreq.c
index 6c88827f4e62..f98f53bf1011 100644
--- a/drivers/cpufreq/tegra186-cpufreq.c
+++ b/drivers/cpufreq/tegra186-cpufreq.c
@@ -65,8 +65,8 @@ struct tegra186_cpufreq_cluster {
 
 struct tegra186_cpufreq_data {
 	void __iomem *regs;
-	struct tegra186_cpufreq_cluster *clusters;
 	const struct tegra186_cpufreq_cpu *cpus;
+	struct tegra186_cpufreq_cluster clusters[];
 };
 
 static int tegra186_cpufreq_init(struct cpufreq_policy *policy)
@@ -221,15 +221,12 @@ static int tegra186_cpufreq_probe(struct platform_device *pdev)
 	struct tegra_bpmp *bpmp;
 	unsigned int i = 0, err;
 
-	data = devm_kzalloc(&pdev->dev, sizeof(*data), GFP_KERNEL);
+	data = devm_kzalloc(&pdev->dev,
+			    struct_size(data, clusters, TEGRA186_NUM_CLUSTERS),
+			    GFP_KERNEL);
 	if (!data)
 		return -ENOMEM;
 
-	data->clusters = devm_kcalloc(&pdev->dev, TEGRA186_NUM_CLUSTERS,
-				      sizeof(*data->clusters), GFP_KERNEL);
-	if (!data->clusters)
-		return -ENOMEM;
-
 	data->cpus = tegra186_cpus;
 
 	bpmp = tegra_bpmp_get(&pdev->dev);
-- 
2.34.1