Re: [PATCH v3 06/13] nvmem: microchip-otpc: add tag-based packet lookup
From: Jonathan Cameron <jic23@kernel.org>
Date: 2026-06-30 23:49:27
Also in:
linux-devicetree, linux-iio, lkml
On Tue, 30 Jun 2026 15:05:56 +0530 Varshini Rajendran [off-list ref] wrote:
Add support for accessing OTP packets by their 4-byte ASCII tag while
preserving backward compatibility with the existing ID-based lookup.
The OTP memory layout can vary across devices and may change over time,
making the packet ID approach unreliable when the memory map is not
known in advance. The packet tag provides a reliable way to identify
and access packets without prior knowledge of the OTP memory layout.
Two offset encoding are now supported:
1. Legacy ID-based: offset = OTP_PKT(id) = id * 4
Used in DT as: reg = <OTP_PKT(1) 76>;
2. TAG-based: offset = 4-byte ASCII packet tag
Used in DT as: reg = <0x41435354 0x4c>; (tag "ACST")
The driver resolves offsets matching valid legacy selectors (multiples
of 4 within the packet count) through ID lookup, falling back to tag
lookup for other values. This ensures existing device trees continue
to work while enabling new tag-based access.
During probe, packet meta data including the tag is read and cached.
The driver also validates OTP memory accessibility and emulation mode
status. When the boot packet is not configured, emulation mode allows
access to the other packets. When both are not available an
informational message is logged.
The stride of the nvmem memory is set to 1 in order to support tag based
offsets, comment in the header file is updated accordingly.
Signed-off-by: Varshini Rajendran <redacted>A couple of trivial things inline Thanks, Jonathan
quoted hunk ↗ jump to hunk
static int mchp_otpc_prepare_read(struct mchp_otpc *otpc, unsigned int offset) {@@ -140,8 +198,29 @@ static int mchp_otpc_prepare_read(struct mchp_otpc *otpc, * offset returned by hardware. * * For this, the read function will return the first requested bytes in the - * packet. The user will have to be aware of the memory footprint before doing - * the read request. + * packet. + * + * Two offset encoding are supported:
"encodings" as there are two of them.
+ * + * 1. Legacy ID-based: offset = OTP_PKT(id) = id * 4 + * Used in DT as: reg = <OTP_PKT(1) 76>; + * 2. TAG-based: offset = 4-byte ASCII packet tag + * Used in DT as: reg = <0x41435354 0x4c>; (tag "ACST") + * + * To use the legacy ID based packet lookup the user will have to be aware of + * the memory footprint before doing the read request. + * + * But by using the TAG based packet lookup, the user won't have to be aware + * of the memory footprint before doing the read request since this driver has + * it abstracted and taken care of. + * + * Practically, there is no way of knowing the mapping of the OTP memory table + * in advance for every device. But by using the packet tag - the identifier + * ASCII value, the packets can be recognized without being aware of the + * flashed OTP memory map table and the payload can be acquired reliably. + * + * While the legacy ID based lookup is still supported, TAG based approach is + * recommended. */
quoted hunk ↗ jump to hunk
@@ -244,7 +356,8 @@ static int mchp_otpc_probe(struct platform_device *pdev) { struct nvmem_device *nvmem; struct mchp_otpc *otpc; - u32 size; + bool emul_enable; + u32 size, tmp;
If possible give tmp a more meaningful name. reg_val or better yet something about which register.
quoted hunk ↗ jump to hunk
int ret; otpc = devm_kzalloc(&pdev->dev, sizeof(*otpc), GFP_KERNEL);@@ -256,10 +369,22 @@ static int mchp_otpc_probe(struct platform_device *pdev) return PTR_ERR(otpc->base); otpc->dev = &pdev->dev; + + tmp = readl_relaxed(otpc->base + MCHP_OTPC_MR); + emul_enable = tmp & MCHP_OTPC_MR_EMUL; + if (emul_enable) + dev_info(otpc->dev, "Emulation mode enabled\n"); + ret = mchp_otpc_init_packets_list(otpc, &size); if (ret) return ret; + if (!size) { + dev_warn(otpc->dev, "Cannot access OTP memory\n"); + if (!emul_enable) + dev_info(otpc->dev, "Boot packet not programmed and emulation mode disabled\n"); + } +