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([2a02:2f05:1204:e000:2cf7:c250:64a9:ae77]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49fcd0ece6esm103469825e9.2.2026.09.19.06.17.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 19 Sep 2026 06:17:18 -0700 (PDT) Message-ID: Date: Sat, 19 Sep 2026 16:17:16 +0300 Precedence: bulk X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 09/17] nvmem: microchip-otpc: nvmem: microchip-otpc: add tag-based packet lookup To: Varshini Rajendran , ehristev@kernel.org, jic23@kernel.org, dlechner@baylibre.com, nuno.sa@analog.com, andy@kernel.org, robh@kernel.org, krzk+dt@kernel.org, conor+dt@kernel.org, nicolas.ferre@microchip.com, alexandre.belloni@bootlin.com, srini@kernel.org, marcelo.schmitt@analog.com, radu.sabau@analog.com, joshua.crofts1@gmail.com, jorge.marques@analog.com, Jonathan.Santos@analog.com, jishnu.prakash@oss.qualcomm.com, antoniu.miclaus@analog.com, duje@dujemihanovic.xyz, mazziesaccount@gmail.com, linux-iio@vger.kernel.org, devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20260806074024.531259-1-varshini.rajendran@microchip.com> <20260806074024.531259-10-varshini.rajendran@microchip.com> Content-Language: en-US From: claudiu beznea In-Reply-To: <20260806074024.531259-10-varshini.rajendran@microchip.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Hi, Varshini, On 8/6/26 10:40, Varshini Rajendran wrote: > Add support for accessing OTP packets by their tag which is a FourCC > 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 encodings are now supported: > 1. Legacy ID-based: offset = OTP_PKT(id) = id * 4 > Used in DT as: reg = ; > 2. TAG-based: offset = 4-byte ASCII packet tag (FourCC) > 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 valid 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 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 > --- > drivers/nvmem/microchip-otpc.c | 117 ++++++++++++++++-- > .../nvmem/microchip,sama7g5-otpc.h | 4 +- > 2 files changed, 110 insertions(+), 11 deletions(-) > > diff --git a/drivers/nvmem/microchip-otpc.c b/drivers/nvmem/microchip-otpc.c > index df979e8549fd..a17f82afdc37 100644 > --- a/drivers/nvmem/microchip-otpc.c > +++ b/drivers/nvmem/microchip-otpc.c > @@ -23,6 +23,8 @@ > #define MCHP_OTPC_SR_READ BIT(6) > #define MCHP_OTPC_HR (0x20) > #define MCHP_OTPC_HR_SIZE GENMASK(15, 8) > +#define MCHP_OTPC_HR_PACKET GENMASK(2, 0) > +#define MCHP_OTPC_HR_PACKET_REGULAR 1 > #define MCHP_OTPC_DR (0x24) > > #define MCHP_OTPC_NAME "mchp-otpc" > @@ -47,11 +49,13 @@ struct mchp_otpc { > * @list: list head > * @id: packet ID > * @offset: packet offset (in words) in OTP memory > + * @tag: 4-byte ASCII (FourCC) tag of the packet > */ > struct mchp_otpc_packet { > struct list_head list; > u32 id; > u32 offset; > + u32 tag; > }; > > static struct mchp_otpc_packet *mchp_otpc_id_to_packet(struct mchp_otpc *otpc, > @@ -70,6 +74,52 @@ static struct mchp_otpc_packet *mchp_otpc_id_to_packet(struct mchp_otpc *otpc, > return NULL; > } > > +static struct mchp_otpc_packet *mchp_otpc_tag_to_packet(struct mchp_otpc *otpc, u32 tag) > +{ > + struct mchp_otpc_packet *packet; > + > + list_for_each_entry(packet, &otpc->packets, list) { > + if (packet->tag == tag) > + return packet; > + } > + > + return NULL; > +} > + > +static bool mchp_otpc_is_valid_fourcc(u32 tag) > +{ > + int i; > + u8 c; > + > + for (i = 0; i < 4; i++) { > + c = (tag >> (i * 8)) & 0xff; > + if (c < 0x20 || c > 0x7e) > + return false; > + } > + > + return true; > +} > + > +static struct mchp_otpc_packet *mchp_otpc_resolve_packet(struct mchp_otpc *otpc, u32 off) > +{ > + /* > + * Legacy id based packet access: offset = id * 4 > + * Inside the driver we use continuous unsigned integer numbers > + * for packet id, thus divide off by 4 before passing it to > + * mchp_otpc_id_to_packet(). > + */ > + u32 remainder = off % 4; > + u32 id = off / 4; > + > + if (!remainder && id < otpc->npackets) > + return mchp_otpc_id_to_packet(otpc, id); > + > + /* > + * TAG-based packet access: offset is a 4-byte ASCII tag (FourCC) > + */ > + return mchp_otpc_tag_to_packet(otpc, off); > +} > + > static int mchp_otpc_prepare_read(struct mchp_otpc *otpc, > unsigned int offset) > { > @@ -140,8 +190,29 @@ 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 encodings are supported: > + * > + * 1. Legacy ID-based: offset = OTP_PKT(id) = id * 4 > + * Used in DT as: reg = ; > + * 2. TAG-based: offset = 4-byte ASCII packet tag (FourCC) > + * 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 (FourCC), 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. > */ > static int mchp_otpc_read(void *priv, unsigned int off, void *val, > size_t bytes) > @@ -154,12 +225,11 @@ static int mchp_otpc_read(void *priv, unsigned int off, void *val, > int ret, payload_size; > > /* > - * We reach this point with off being multiple of stride = 4 to > - * be able to cross the subsystem. Inside the driver we use continuous > - * unsigned integer numbers for packet id, thus divide off by 4 > - * before passing it to mchp_otpc_id_to_packet(). > + * From this point the offset has to be translated into the actual > + * packet. For this we traverse the table of contents stored in a list > + * "packet" based on the access type - packet id or tag. > */ > - packet = mchp_otpc_id_to_packet(otpc, off / 4); > + packet = mchp_otpc_resolve_packet(otpc, off); > if (!packet) > return -EINVAL; > offset = packet->offset; > @@ -190,10 +260,25 @@ static int mchp_otpc_read(void *priv, unsigned int off, void *val, > return 0; > } > > +static int mchp_otpc_read_packet_tag(struct mchp_otpc *otpc, unsigned int offset, > + unsigned int *tag) > +{ > + int ret; > + > + ret = mchp_otpc_prepare_read(otpc, offset); Is this needed? At the moment this function is called a prepared was already done (in mchp_otpc_init_packets_list()). The header was read. As of the current code from mchp_otpc_read(), after that, writing 0 (for DR increment) to AR and then reading DR leads to successfully reading the DR. However, the manual [1] mentions the following on step 4 from chapter 37.5.3.5. Read: 4/ Read the header of the packet in the OTPC_HR register. To read each payload word, the *address of the payload* word must be written in OTPC_AR.DADDR. The payload word is then available in OTPC_DR. If OTPC_AR.INCRT is set to AFTER_READ, any read in the OTPC_DR increments the DADDR field. So, maybe there is a bug in the current code, in the mcp_otpc_read() and this function, and instead of writing 0 to AR, after reading the header we need to write the AR.ADDR and INCR=0 and after that read the DR? And the HW happens to behave correctly even with the current implemented approach? [1] https://ww1.microchip.com/downloads/aemDocuments/documents/MPU32/ProductDocuments/DataSheets/SAMA7D6-Series-Data-Sheet-DS60001851.pdf > + if (ret) > + return ret; > + > + writel_relaxed(0, otpc->base + MCHP_OTPC_AR); > + *tag = readl_relaxed(otpc->base + MCHP_OTPC_DR); > + > + return 0; > +} > + > static int mchp_otpc_init_packets_list(struct mchp_otpc *otpc, u32 *size) > { > struct mchp_otpc_packet *packet; > - u32 word, word_pos = 0, id = 0, npackets = 0, payload_size; > + u32 word, word_pos = 0, id = 0, npackets = 0, payload_size, type; > int ret; > > INIT_LIST_HEAD(&otpc->packets); > @@ -215,6 +300,20 @@ static int mchp_otpc_init_packets_list(struct mchp_otpc *otpc, u32 *size) > > packet->id = id++; > packet->offset = word_pos; > + type = FIELD_GET(MCHP_OTPC_HR_PACKET, word); > + > + if (type == MCHP_OTPC_HR_PACKET_REGULAR) { > + ret = mchp_otpc_read_packet_tag(otpc, packet->offset, > + &packet->tag); > + if (ret) > + return ret; > + > + if (!mchp_otpc_is_valid_fourcc(packet->tag)) > + packet->tag = 0; > + } else { > + packet->tag = 0; > + } The else path is not needed since packet is allocated with devm_kzalloc(). With that, if the above is true this section could be written as: if (type == MCHP_OTPC_HR_PACKET_REGULAR) { u32 tag; writel_relaxed(packet->offset + 4, otpc->base + MCHP_OTPC_AR); // or if the manual is wrong: // writel_relaxed(0, otpc->base + MCHP_OTPC_AR); tag = readl_relaxed(otpc->base + MCHP_OTPC_DR); if (mchp_otpc_is_valid_fourcc(tag)) packet->tag = tag; } Thank you, Claudiu